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Date: Tue, 27 Aug 2019 07:53:51 +0200
From: Juergen Gross <jgross@...e.com>
To: Nadav Amit <namit@...are.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Andy Lutomirski <luto@...nel.org>,
StefanoStabellini <sstabellini@...nel.org>,
the arch/x86 maintainers <x86@...nel.org>,
Thomas Gleixner <tglx@...utronix.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
xen-devel <xen-devel@...ts.xenproject.org>,
Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Ingo Molnar <mingo@...hat.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH v2 1/3] x86/mm/tlb: Change __flush_tlb_one_user
interface
On 26.08.19 18:38, Nadav Amit wrote:
>> On Aug 26, 2019, at 12:51 AM, Juergen Gross <jgross@...e.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@...cle.com>
>>> Cc: Juergen Gross <jgross@...e.com>
>>> Cc: Stefano Stabellini <sstabellini@...nel.org>
>>> Cc: xen-devel@...ts.xenproject.org
>>> Signed-off-by: Nadav Amit <namit@...are.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?
Yes, it does.
BTW, preempt_disable/enable() is needed as xen_mc_batch() ...
xen_mc_issue() is using a percpu buffer.
Juergen
>
> ---
> 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,
>
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