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SubjectRe: [RFC PATCH v2 1/3] x86/mm/tlb: Change __flush_tlb_one_user interface
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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.


Juergen

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