linux-kernel - Re: [PATCH v3 2/5] arch/x86: Refactor tlbflush and l1d flush

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Message-ID: <b34460c972d7c862c2a390381ace5e689c779a38.camel@amazon.com>
Date:   Fri, 17 Apr 2020 22:58:44 +0000
From:   "Singh, Balbir" <sblbir@...zon.com>
To:     "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:     "keescook@...omium.org" <keescook@...omium.org>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "benh@...nel.crashing.org" <benh@...nel.crashing.org>,
        "jpoimboe@...hat.com" <jpoimboe@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>
Subject: Re: [PATCH v3 2/5] arch/x86: Refactor tlbflush and l1d flush

On Fri, 2020-04-17 at 15:03 +0200, Thomas Gleixner wrote:
> 
> Balbir Singh <sblbir@...zon.com> writes:
> > +void populate_tlb_with_flush_pages(void *l1d_flush_pages);
> > +void flush_l1d_cache_sw(void *l1d_flush_pages);
> > +int flush_l1d_cache_hw(void);
> 
> l1d_flush_populate_pages();
> l1d_flush_sw()
> l1d_flush_hw()
> 
> Hmm?
> 

I can rename them

> > +void populate_tlb_with_flush_pages(void *l1d_flush_pages)
> > +{
> > +     int size = PAGE_SIZE << L1D_CACHE_ORDER;
> > +
> > +     asm volatile(
> > +             /* First ensure the pages are in the TLB */
> > +             "xorl   %%eax, %%eax\n"
> > +             ".Lpopulate_tlb:\n\t"
> > +             "movzbl (%[flush_pages], %%" _ASM_AX "), %%ecx\n\t"
> > +             "addl   $4096, %%eax\n\t"
> > +             "cmpl   %%eax, %[size]\n\t"
> > +             "jne    .Lpopulate_tlb\n\t"
> > +             "xorl   %%eax, %%eax\n\t"
> > +             "cpuid\n\t"
> > +             :: [flush_pages] "r" (l1d_flush_pages),
> > +                 [size] "r" (size)
> > +             : "eax", "ebx", "ecx", "edx");
> > +}
> > +EXPORT_SYMBOL_GPL(populate_tlb_with_flush_pages);
> 
> I probably missed the fine print in the change log why this is separate
> from the SW flush function.

In the RFC we discussed if we really need to prefetch the pages into the TLB 
prior to the flush and I pointed out or thought that the TLB prefetch was not
required for these patches (L1D flush), so I split it out.

> 
> > +int flush_l1d_cache_hw(void)
> > +{
> > +     if (static_cpu_has(X86_FEATURE_FLUSH_L1D)) {
> > +             wrmsrl(MSR_IA32_FLUSH_CMD, L1D_FLUSH);
> > +             return 0;
> > +     }
> > +     return -ENOTSUPP;
> > +}
> > +EXPORT_SYMBOL_GPL(flush_l1d_cache_hw);
> 
> along with the explanation why this needs to be two functions.
> 

Are you suggesting I abstract the hw and sw flushes into one function? I can
do that.

> > -     if (static_cpu_has(X86_FEATURE_FLUSH_L1D)) {
> > -             wrmsrl(MSR_IA32_FLUSH_CMD, L1D_FLUSH);
> > +     if (flush_l1d_cache_hw() == 0)
> >               return;
> > -     }
> 
>         if (!l1d_flush_hw())
>                 return;
> 
> > -     asm volatile(
> > -             /* First ensure the pages are in the TLB */
> > -             "xorl   %%eax, %%eax\n"
> > -             ".Lpopulate_tlb:\n\t"
> > -             "movzbl (%[flush_pages], %%" _ASM_AX "), %%ecx\n\t"
> > -             "addl   $4096, %%eax\n\t"
> > -             "cmpl   %%eax, %[size]\n\t"
> > -             "jne    .Lpopulate_tlb\n\t"
> > -             "xorl   %%eax, %%eax\n\t"
> > -             "cpuid\n\t"
> > -             /* Now fill the cache */
> > -             "xorl   %%eax, %%eax\n"
> > -             ".Lfill_cache:\n"
> > -             "movzbl (%[flush_pages], %%" _ASM_AX "), %%ecx\n\t"
> > -             "addl   $64, %%eax\n\t"
> > -             "cmpl   %%eax, %[size]\n\t"
> > -             "jne    .Lfill_cache\n\t"
> > -             "lfence\n"
> > -             :: [flush_pages] "r" (vmx_l1d_flush_pages),
> > -                 [size] "r" (size)
> > -             : "eax", "ebx", "ecx", "edx");
> > +     preempt_disable();
> > +     populate_tlb_with_flush_pages(vmx_l1d_flush_pages);
> > +     flush_l1d_cache_sw(vmx_l1d_flush_pages);
> > +     preempt_enable();
> 
> The preempt_disable/enable was not there before, right? Why do we need
> that now? If this is a fix, then that should be a separate patch.
> 

No they were not, I added them because I was concerned about preemption, it's
a speculative change, my concern was that we could fill the TLB and then get
preempted. Looking at the caller context, we do run with interrupts disabled,
I might have been too conservative, we don't need this. I'll remove it

Balbir