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SubjectRe: [RFC PATCH v2 1/3] x86/mm/tlb: Change __flush_tlb_one_user interface
Date
> On Aug 26, 2019, at 12:51 AM, Juergen Gross <jgross@suse.com> wrote:
>
> On 24.08.19 00:52, Nadav Amit wrote:
>> __flush_tlb_one_user() currently flushes a single entry, and flushes it
>> both in the kernel and user page-tables, when PTI is enabled.
>> Change __flush_tlb_one_user() and related interfaces into
>> __flush_tlb_range() that flushes a range and does not flush the user
>> page-table.
>> This refactoring is needed for the next patch, but regardless makes
>> sense and has several advantages. First, only Xen-PV, which does not
>> use PTI, implements the paravirtual interface of flush_tlb_one_user() so
>> nothing is broken by separating the user and kernel page-table flushes,
>> and the interface is more intuitive.
>> Second, INVLPG can flush unrelated mappings, and it is also a
>> serializing instruction. It is better to have a tight loop that flushes
>> the entries.
>> Third, currently __flush_tlb_one_kernel() also flushes the user
>> page-tables, which is not needed. This allows to avoid this redundant
>> flush.
>> Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>> Cc: Juergen Gross <jgross@suse.com>
>> Cc: Stefano Stabellini <sstabellini@kernel.org>
>> Cc: xen-devel@lists.xenproject.org
>> Signed-off-by: Nadav Amit <namit@vmware.com>
>> ---
>> arch/x86/include/asm/paravirt.h | 5 ++--
>> arch/x86/include/asm/paravirt_types.h | 3 ++-
>> arch/x86/include/asm/tlbflush.h | 24 +++++------------
>> arch/x86/kernel/paravirt.c | 7 ++---
>> arch/x86/mm/tlb.c | 39 ++++++++++++++++++++++-----
>> arch/x86/xen/mmu_pv.c | 21 +++++++++------
>> 6 files changed, 62 insertions(+), 37 deletions(-)
>
> ...
>
>> diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
>> index 48f7c7eb4dbc..ed68657f5e77 100644
>> --- a/arch/x86/xen/mmu_pv.c
>> +++ b/arch/x86/xen/mmu_pv.c
>> @@ -1325,22 +1325,27 @@ static noinline void xen_flush_tlb(void)
>> preempt_enable();
>> }
>> -static void xen_flush_tlb_one_user(unsigned long addr)
>> +static void xen_flush_tlb_range(unsigned long start, unsigned long end,
>> + u8 stride_shift)
>> {
>> struct mmuext_op *op;
>> struct multicall_space mcs;
>> -
>> - trace_xen_mmu_flush_tlb_one_user(addr);
>> + unsigned long addr;
>> preempt_disable();
>> mcs = xen_mc_entry(sizeof(*op));
>> op = mcs.args;
>> - op->cmd = MMUEXT_INVLPG_LOCAL;
>> - op->arg1.linear_addr = addr & PAGE_MASK;
>> - MULTI_mmuext_op(mcs.mc, op, 1, NULL, DOMID_SELF);
>> - xen_mc_issue(PARAVIRT_LAZY_MMU);
>> + for (addr = start; addr < end; addr += 1ul << stride_shift) {
>> + trace_xen_mmu_flush_tlb_one_user(addr);
>> +
>> + op->cmd = MMUEXT_INVLPG_LOCAL;
>> + op->arg1.linear_addr = addr & PAGE_MASK;
>> + MULTI_mmuext_op(mcs.mc, op, 1, NULL, DOMID_SELF);
>> +
>> + xen_mc_issue(PARAVIRT_LAZY_MMU);
>> + }
>
> For this kind of usage (a loop) you should:
>
> - replace the call of xen_mc_entry() with xen_mc_batch()
> - use xen_extend_mmuext_op() for each loop iteration
> - call xen_mc_issue() after the loop
>
> Additionally I'd like you to replace trace_xen_mmu_flush_tlb_one_user()
> with trace_xen_mmu_flush_tlb_range() taking all three parameters and
> keep it where it was (out of the loop).
>
> The paravirt parts seem to be okay.

Thanks for the quick response. I don’t think the preempt_disable/enable() is
needed in this case, but I kept them. Does the following match what you had
in mind?

---
arch/x86/xen/mmu_pv.c | 25 ++++++++++++++-----------
include/trace/events/xen.h | 18 ++++++++++++------
2 files changed, 26 insertions(+), 17 deletions(-)

diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 48f7c7eb4dbc..faa01591df36 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c
@@ -1325,20 +1325,23 @@ static noinline void xen_flush_tlb(void)
preempt_enable();
}

-static void xen_flush_tlb_one_user(unsigned long addr)
+static void xen_flush_tlb_range(unsigned long start, unsigned long end,
+ u8 stride_shift)
{
- struct mmuext_op *op;
- struct multicall_space mcs;
-
- trace_xen_mmu_flush_tlb_one_user(addr);
+ struct mmuext_op op;
+ unsigned long addr;

preempt_disable();

- mcs = xen_mc_entry(sizeof(*op));
- op = mcs.args;
- op->cmd = MMUEXT_INVLPG_LOCAL;
- op->arg1.linear_addr = addr & PAGE_MASK;
- MULTI_mmuext_op(mcs.mc, op, 1, NULL, DOMID_SELF);
+ xen_mc_batch();
+ op.cmd = MMUEXT_INVLPG_LOCAL;
+
+ trace_xen_mmu_flush_tlb_range(start, end, stride_shift);
+
+ for (addr = start; addr < end; addr += 1ul << stride_shift) {
+ op.arg1.linear_addr = addr & PAGE_MASK;
+ xen_extend_mmuext_op(&op);
+ }

xen_mc_issue(PARAVIRT_LAZY_MMU);

@@ -2394,7 +2397,7 @@ static const struct pv_mmu_ops xen_mmu_ops __initconst = {

.flush_tlb_user = xen_flush_tlb,
.flush_tlb_kernel = xen_flush_tlb,
- .flush_tlb_one_user = xen_flush_tlb_one_user,
+ .flush_tlb_range = xen_flush_tlb_range,
.flush_tlb_multi = xen_flush_tlb_multi,
.tlb_remove_table = tlb_remove_table,

diff --git a/include/trace/events/xen.h b/include/trace/events/xen.h
index 546022acf160..32bf0a94b4d8 100644
--- a/include/trace/events/xen.h
+++ b/include/trace/events/xen.h
@@ -352,14 +352,20 @@ DECLARE_EVENT_CLASS(xen_mmu_pgd,
DEFINE_XEN_MMU_PGD_EVENT(xen_mmu_pgd_pin);
DEFINE_XEN_MMU_PGD_EVENT(xen_mmu_pgd_unpin);

-TRACE_EVENT(xen_mmu_flush_tlb_one_user,
- TP_PROTO(unsigned long addr),
- TP_ARGS(addr),
+TRACE_EVENT(xen_mmu_flush_tlb_range,
+ TP_PROTO(unsigned long start, unsigned long end,
+ unsigned char stride_shift),
+ TP_ARGS(start, end, stride_shift),
TP_STRUCT__entry(
- __field(unsigned long, addr)
+ __field(unsigned long, start)
+ __field(unsigned long, end)
+ __field(unsigned char, stride_shift)
),
- TP_fast_assign(__entry->addr = addr),
- TP_printk("addr %lx", __entry->addr)
+ TP_fast_assign(__entry->start = start;
+ __entry->end = end;
+ __entry->stride_shift = stride_shift),
+ TP_printk("start %lx end %lx stride_shift %d", __entry->start,
+ __entry->end, __entry->stride_shift)
);

TRACE_EVENT(xen_mmu_flush_tlb_multi,
--
2.17.1
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