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SubjectRE: [PATCH] [RFC] pmem: add pmem support codes on ARM64
Date
Hi Mark,

Thanks a lot for your detailed comments!
I'll try to answer and fix the points below and let me get back when they are resolved.

Best Regards,
Kwangwoo Lee

> -----Original Message-----
> From: Mark Rutland [mailto:mark.rutland@arm.com]
> Sent: Tuesday, July 05, 2016 7:32 PM
> To: 이광우(LEE KWANGWOO) MS SW
> Cc: linux-arm-kernel@lists.infradead.org; linux-nvdimm@lists.01.org; Catalin Marinas; Will Deacon;
> Ross Zwisler; Dan Williams; Vishal Verma; 김현철(KIM HYUNCHUL) MS SW; linux-kernel@vger.kernel.org; 정우
> 석(CHUNG WOO SUK) MS SW
> Subject: Re: [PATCH] [RFC] pmem: add pmem support codes on ARM64
>
> Hi,
>
> On Tue, Jul 05, 2016 at 04:31:17PM +0900, Kwangwoo Lee wrote:
> > The PMEM driver on top of NVDIMM(Non-Volatile DIMM) has already been
> > supported on X86_64 and there exist several ARM64 platforms which support
> > DIMM type memories.
> >
> > This patch set enables the PMEM driver on ARM64 (AArch64) architecture
> > on top of NVDIMM. While developing this patch set, QEMU 2.6.50 with NVDIMM
> > emulation for ARM64 has also been developed and tested on it.
>
> Just to check, which platforms is this intended to support?

Currently it is under an evaluation stage using QEMU VIRT platform which was
patched to support NVDIMM and ACPI(NFIT). The features - NVDIMM, ACPI(NFIT)-
have already been implemented on X86 and they were mostly reused for AAarch64
in the patch set.

The reason that I choose the platform is to get a fast development and debugging
cycle. The way was introduced in http://vmsplice.net/~stefan/nvdimm_slides_public.pdf
by Marc Mari Barcelo <markmb@redhat.com>.

> Architectural support for persistent memory (the addition of the
> Point-of-Persistency, along with the new DC CVAP instruction) came in
> ARMv8.2, and prior to this I'm not entirely sure how things are
> expected/likely to work (i.e. I'm not sure whether maintenance to the
> PoC is sufficient).

I was not aware of this DC CVAP (Point-of-Persistency) introduced on ARMv8.2
and seems to be similar one with PCOMMIT on X86. I'll investigate more on this
which can be handled in the next round. Thanks for the information!

> >
> > $ dmesg
> > [ 0.000000] Booting Linux on physical CPU 0x0
> > [ 0.000000] Linux version 4.6.0-rc2kw-dirty (kwangwoo@VBox15) (gcc version 5.2.1 20151010 (Ubuntu
> 5.2.1-22ubuntu1) ) #10 SMP Tue Jul 5 11:30:33 KST 2016
> > [ 0.000000] Boot CPU: AArch64 Processor [411fd070]
> > [ 0.000000] efi: Getting EFI parameters from FDT:
> > [ 0.000000] EFI v2.60 by EDK II
> > [ 0.000000] efi: SMBIOS 3.0=0x58710000 ACPI 2.0=0x589b0000
> > [ 0.000000] ACPI: Early table checksum verification disabled
> > [ 0.000000] ACPI: RSDP 0x00000000589B0000 000024 (v02 BOCHS )
> > [ 0.000000] ACPI: XSDT 0x00000000589A0000 00005C (v01 BOCHS BXPCFACP 00000001 01000013)
> > [ 0.000000] ACPI: FACP 0x0000000058620000 00010C (v05 BOCHS BXPCFACP 00000001 BXPC 00000001)
> > [ 0.000000] ACPI: DSDT 0x0000000058630000 00108F (v02 BOCHS BXPCDSDT 00000001 BXPC 00000001)
> > [ 0.000000] ACPI: APIC 0x0000000058610000 0000A8 (v03 BOCHS BXPCAPIC 00000001 BXPC 00000001)
> > [ 0.000000] ACPI: GTDT 0x0000000058600000 000060 (v02 BOCHS BXPCGTDT 00000001 BXPC 00000001)
> > [ 0.000000] ACPI: MCFG 0x00000000585F0000 00003C (v01 BOCHS BXPCMCFG 00000001 BXPC 00000001)
> > [ 0.000000] ACPI: SPCR 0x00000000585E0000 000050 (v02 BOCHS BXPCSPCR 00000001 BXPC 00000001)
> > [ 0.000000] ACPI: NFIT 0x00000000585D0000 0000E0 (v01 BOCHS BXPCNFIT 00000001 BXPC 00000001)
> > [ 0.000000] ACPI: SSDT 0x00000000585C0000 000131 (v01 BOCHS NVDIMM 00000001 BXPC 00000001)
> > ...
> > [ 5.386743] pmem0: detected capacity change from 0 to 1073741824
> > ...
> > [ 531.952466] EXT4-fs (pmem0): DAX enabled. Warning: EXPERIMENTAL, use at your own risk
> > [ 531.961073] EXT4-fs (pmem0): mounted filesystem with ordered data mode. Opts: dax
> >
> > $ mount
> > rootfs on / type rootfs (rw,size=206300k,nr_inodes=51575)
> > ...
> > /dev/pmem0 on /mnt/mem type ext4 (rw,relatime,dax,data=ordered)
> >
> > $ df -h
> > Filesystem Size Used Available Use% Mounted on
> > ...
> > /dev/pmem0 975.9M 1.3M 907.4M 0% /mnt/mem
> >
> > Signed-off-by: Kwangwoo Lee <kwangwoo.lee@sk.com>
> > ---
> > arch/arm64/Kconfig | 2 +
> > arch/arm64/include/asm/cacheflush.h | 2 +
> > arch/arm64/include/asm/pmem.h | 149 ++++++++++++++++++++++++++++++++++++
> > 3 files changed, 153 insertions(+)
> > create mode 100644 arch/arm64/include/asm/pmem.h
> >
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index 4f43622..ee1d679 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -15,6 +15,8 @@ config ARM64
> > select ARCH_WANT_COMPAT_IPC_PARSE_VERSION
> > select ARCH_WANT_FRAME_POINTERS
> > select ARCH_HAS_UBSAN_SANITIZE_ALL
> > + select ARCH_HAS_PMEM_API
> > + select ARCH_HAS_MMIO_FLUSH
> > select ARM_AMBA
> > select ARM_ARCH_TIMER
> > select ARM_GIC
> > diff --git a/arch/arm64/include/asm/cacheflush.h b/arch/arm64/include/asm/cacheflush.h
> > index c64268d..113bd1f 100644
> > --- a/arch/arm64/include/asm/cacheflush.h
> > +++ b/arch/arm64/include/asm/cacheflush.h
> > @@ -133,6 +133,8 @@ static inline void __flush_icache_all(void)
> > */
> > #define flush_icache_page(vma,page) do { } while (0)
> >
> > +#define mmio_flush_range(addr, size) __flush_dcache_area(addr, size)
>
> What exactly is mmio_flush_range() intended to do? Does it need to
> ensure that data is written back to memory (but not necessarily
> persistently)?
>
> What mappings is this used for?

If some parts of NVDIMM are used with block region, not pmem region, and a block io read
fromwith NFIT_BLK_READ_FLUSH flag set mmio_flush_range() is called. In short, the function
can be called when a block io read from block region in NVDIMM.

When I look at the clflush_cache_range() on X86, it only use mb() and CLFLUSHOPT, I think that
it does not require persistency in that point. Furthermore, if NVDIMM is used as pmem region,
then the function is not called. In such a case, it's for the build completion of nfit.c.

If I missed something, please let me know.

> > +
> > /*
> > * Not required on AArch64 (PIPT or VIPT non-aliasing D-cache).
> > */
> > diff --git a/arch/arm64/include/asm/pmem.h b/arch/arm64/include/asm/pmem.h
> > new file mode 100644
> > index 0000000..4bdf015
> > --- /dev/null
> > +++ b/arch/arm64/include/asm/pmem.h
> > @@ -0,0 +1,149 @@
> > +/*
> > + * Based on arch/x86/include/asm/pmem.h
> > + *
> > + * Copyright(c) 2016 SK hynix Inc. Kwangwoo Lee <kwangwoo.lee@sk.com>
> > + *
> > + * This program is free software; you can redistribute it and/or modify
> > + * it under the terms of version 2 of the GNU General Public License as
> > + * published by the Free Software Foundation.
> > + *
> > + * This program is distributed in the hope that it will be useful, but
> > + * WITHOUT ANY WARRANTY; without even the implied warranty of
> > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> > + * General Public License for more details.
> > + */
> > +#ifndef __ASM_ARM64_PMEM_H__
> > +#define __ASM_ARM64_PMEM_H__
>
> Nit: make this __ASM_PMEM_H, to match the style of other arm64 headers

I'll change this with __ASM_PMEM_H.

> > +
> > +#include <linux/uaccess.h>
> > +#include <asm/cacheflush.h>
> > +
> > +#ifdef CONFIG_ARCH_HAS_PMEM_API
>
> Can we not put the headers under this guard?

OK.

> > +/**
> > + * arch_memcpy_to_pmem - copy data to persistent memory
> > + * @dst: destination buffer for the copy
> > + * @src: source buffer for the copy
> > + * @n: length of the copy in bytes
> > + *
> > + * Copy data to persistent memory media via non-temporal stores so that
> > + * a subsequent arch_wmb_pmem() can flush cpu and memory controller
> > + * write buffers to guarantee durability.
> > + */
>
> This comment appears to be a direct copy of the x86 comment, and I don't
> think it's correct. I don't think non-temporal stores come into this at
> all for arm64.

Sorry! I'll fix the comments carefully which appropriate for ARM64.

> > +static inline void arch_memcpy_to_pmem(void __pmem *dst, const void *src,
> > + size_t n)
> > +{
> > + int unwritten;
> > +
> > + /*
> > + * We are copying between two kernel buffers, if
> > + * __copy_from_user_inatomic_nocache() returns an error (page
> > + * fault) we would have already reported a general protection fault
> > + * before the WARN+BUG.
> > + */
> > + unwritten = __copy_from_user_inatomic_nocache((void __force *) dst,
> > + (void __user *) src, n);
>
> We didn't define ARCH_HAS_NOCACHE_UACCESS, so this is just
> __copy_from_user_inatomic, no?

Yes, you are right.

> What is the expectation regarding how persistent memory is mapped?

LIBNVDIMM subsystem provides three types of NVDIMMs, PMEM, BLK and mixed
from the Documentation/nvdimm/nvdimm.txt

PMEM: A system-physical-address range where writes are persistent. A
block device composed of PMEM is capable of DAX. A PMEM address range
may span an interleave of several DIMMs.

BLK: A set of one or more programmable memory mapped apertures provided
by a DIMM to access its media. This indirection precludes the
performance benefit of interleaving, but enables DIMM-bounded failure
modes.

> I'm not sure I follow why we use __copy_from_user_inatomic here, if the
> src is a kernel buffer. Why do we expect faults on kernel addresses?
>
> Do callers set up KERNEL_DS appropriately before calling
> __copy_from_user?

Hum.. it looks that most of calls from submit_bio()->generic_make_request()
under pmem block layer. If then, can we assume that the dst and src are
in KERNEL_DS and no possibility to cause a fault?

The above point is not clear to me. I need to investigate more on this or
it'll be appreciate that someone else can guide to me.

> > + if (WARN(unwritten, "%s: fault copying %p <- %p unwritten: %d\n",
> > + __func__, dst, src, unwritten))
> > + BUG();
> > +}
> > +
> > +static inline int arch_memcpy_from_pmem(void *dst, const void __pmem *src,
> > + size_t n)
> > +{
> > + memcpy(dst, (void __force *) src, n);
> > + return 0;
> > +}
>
> We don't need to do any special checks here?

The code of MCE(Machine Check Exception) on X86 is not considered here.

> The asymmetry doesn't seem right to me. What am I missing?

Let me check this again.

> > +/**
> > + * arch_wmb_pmem - synchronize writes to persistent memory
> > + *
> > + * After a series of arch_memcpy_to_pmem() operations this drains data
> > + * from cpu write buffers and any platform (memory controller) buffers
> > + * to ensure that written data is durable on persistent memory media.
> > + */
> > +static inline void arch_wmb_pmem(void)
> > +{
> > + /*
> > + * PCOMMIT instruction only exists on x86. So pcommit_sfence() has been
> > + * removed after wmb(). Note, that we've already arranged for pmem
> > + * writes to avoid the cache via arch_memcpy_to_pmem().
> > + */
> > + wmb();
> > +}
>
> In ARMv8.2, I believe that a DSB following a series of DC CVAP
> instructions is sufficient to ensure that data has made it to the point
> of persistency.

OK. Thanks for the information. I agree that this point is the right point to
ensure the point of persistence since after finishing the write operation this
function is called.

> Prior to that, I'm not sure whether a DSB following a series of DC CVAC
> will be sufficient on all platforms.

DC CVAC requires address range.. It seems that the range handling need to be
considered appropriately here..

> > +
> > +/**
> > + * arch_wb_cache_pmem - write back a cache range
> > + * @vaddr: virtual start address
> > + * @size: number of bytes to write back
> > + *
> > + * Write back a cache range. This function requires explicit ordering with an
> > + * arch_wmb_pmem() call.
> > + */
> > +static inline void arch_wb_cache_pmem(void __pmem *addr, size_t size)
> > +{
> > + __clean_dcache_area_pou(addr, size);
> > +}
>
> Below (per arch_clear_pmem), it looks like this is required to force
> data to be written back to the Point-of-Persistency.
>
> Cleaning to the PoU is not appropriate, as that only guarantees
> visibility to instruction fetches, and does not guarantee that data was
> written back to memory or to the Point-of-Persistency.

The original arch_wb_cache_pmem() on X86 uses CLWB which does not invalidate and
just clean cache from my understanding. To maintain the point of persistency, the
write point seems to be arch_wmb_pmem() in my opinion..

> I believe that with ARMv8.2 this needs to consist of DC CVAP (i.e. we
> need a __clean_dache_area_pop).

I could not find the ARMv8.2 document which describes DC CVAP in infocenter.arm.com.
Is it released on public? I could only find ARMv8.1 Reference Manual.

> > +/*
> > + * copy_from_iter_nocache() only uses non-temporal stores for iovec iterators,
> > + * so for other types (bvec & kvec) we must do a cache write-back.
> > + */
> > +static inline bool __iter_needs_pmem_wb(struct iov_iter *i)
> > +{
> > + return iter_is_iovec(i) == false;
> > +}
>
> As previously, the copy+pasted x86 comment is meaningless here. I don't
> understand what this is intended to achieve.

Let me check again.

> > +/**
> > + * arch_copy_from_iter_pmem - copy data from an iterator to PMEM
> > + * @addr: PMEM destination address
> > + * @bytes: number of bytes to copy
> > + * @i: iterator with source data
> > + *
> > + * Copy data from the iterator 'i' to the PMEM buffer starting at 'addr'.
> > + * This function requires explicit ordering with an arch_wmb_pmem() call.
> > + */
> > +static inline size_t arch_copy_from_iter_pmem(void __pmem *addr, size_t bytes,
> > + struct iov_iter *i)
> > +{
> > + void *vaddr = (void __force *)addr;
> > + size_t len;
> > +
> > + /* TODO: skip the write-back by always using non-temporal stores */
>
> Copy+pasted comment again. This is not relevant for arm64.

OK. I'll fix with appropriate comments.

> > + len = copy_from_iter_nocache(vaddr, bytes, i);
> > +
> > + if (__iter_needs_pmem_wb(i))
> > + arch_wb_cache_pmem(addr, bytes);
> > +
> > + return len;
> > +}
> > +
> > +/**
> > + * arch_clear_pmem - zero a PMEM memory range
> > + * @addr: virtual start address
> > + * @size: number of bytes to zero
> > + *
> > + * Write zeros into the memory range starting at 'addr' for 'size' bytes.
> > + * This function requires explicit ordering with an arch_wmb_pmem() call.
> > + */
> > +static inline void arch_clear_pmem(void __pmem *addr, size_t size)
> > +{
> > + void *vaddr = (void __force *)addr;
> > +
> > + memset(vaddr, 0, size);
> > + arch_wb_cache_pmem(addr, size);
> > +}
> > +
> > +static inline void arch_invalidate_pmem(void __pmem *addr, size_t size)
> > +{
> > + /* barrier before clean and invalidate */
> > + mb();
>
> That comment is not very helpful.
>
> What exactly are we trying to order before the __flush_dcache_area()
> call?

OK. I'll check again.

> > +
> > + __flush_dcache_area(addr, size);
> > +}
> > +
> > +static inline bool __arch_has_wmb_pmem(void)
> > +{
> > + return true;
> > +}
>
> I'm not sure that we should do this in the absence of the ARMv8.2
> extensions for persistent memory, as I don't know whether we can
> guarantee durability.

It seems that it is required to check ARM architecture version here
(before/after ARMv8.2) to respond the correct state.

Thank you very much for your time to review this!

> Thanks,
> Mark.
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