linux-kernel - Re: [PATCH v4 2/6] arch/x86/kvm: Refactor tlbflush and l1d flush

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Message-ID: <871aed902ea0932288c47059771aa8624db55907.camel@amazon.com>
Date:   Sat, 25 Apr 2020 02:00:07 +0000
From:   "Singh, Balbir" <sblbir@...zon.com>
To:     "thomas.lendacky@....com" <thomas.lendacky@....com>,
        "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 v4 2/6] arch/x86/kvm: Refactor tlbflush and l1d flush

On Fri, 2020-04-24 at 14:04 -0500, Tom Lendacky wrote:
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> 
> 
> 
> On 4/23/20 9:01 AM, Balbir Singh wrote:
> > Refactor the existing assembly bits into smaller helper functions
> > and also abstract L1D_FLUSH into a helper function. Use these
> > functions in kvm for L1D flushing.
> > 
> > Reviewed-by: Kees Cook <keescook@...omium.org>
> > Signed-off-by: Balbir Singh <sblbir@...zon.com>
> > ---
> >   arch/x86/include/asm/cacheflush.h |  3 ++
> >   arch/x86/kernel/l1d_flush.c       | 54 +++++++++++++++++++++++++++++++
> >   arch/x86/kvm/vmx/vmx.c            | 29 ++---------------
> >   3 files changed, 60 insertions(+), 26 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/cacheflush.h
> > b/arch/x86/include/asm/cacheflush.h
> > index bac56fcd9790..21cc3b28fa63 100644
> > --- a/arch/x86/include/asm/cacheflush.h
> > +++ b/arch/x86/include/asm/cacheflush.h
> > @@ -8,7 +8,10 @@
> > 
> >   #define L1D_CACHE_ORDER 4
> >   void clflush_cache_range(void *addr, unsigned int size);
> > +void l1d_flush_populate_tlb(void *l1d_flush_pages);
> >   void *l1d_flush_alloc_pages(void);
> >   void l1d_flush_cleanup_pages(void *l1d_flush_pages);
> > +void l1d_flush_sw(void *l1d_flush_pages);
> > +int l1d_flush_hw(void);
> > 
> >   #endif /* _ASM_X86_CACHEFLUSH_H */
> > diff --git a/arch/x86/kernel/l1d_flush.c b/arch/x86/kernel/l1d_flush.c
> > index d605878c8f28..5871794f890d 100644
> > --- a/arch/x86/kernel/l1d_flush.c
> > +++ b/arch/x86/kernel/l1d_flush.c
> > @@ -34,3 +34,57 @@ void l1d_flush_cleanup_pages(void *l1d_flush_pages)
> >       free_pages((unsigned long)l1d_flush_pages, L1D_CACHE_ORDER);
> >   }
> >   EXPORT_SYMBOL_GPL(l1d_flush_cleanup_pages);
> > +
> > +/*
> > + * Not all users of l1d flush would want to populate the TLB first
> > + * split out the function so that callers can optionally flush the L1D
> > + * cache via sw without prefetching the TLB.
> > + */
> > +void l1d_flush_populate_tlb(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(l1d_flush_populate_tlb);
> > +
> > +int l1d_flush_hw(void)
> > +{
> > +     if (static_cpu_has(X86_FEATURE_FLUSH_L1D)) {
> > +             wrmsrl(MSR_IA32_FLUSH_CMD, L1D_FLUSH);
> > +             return 0;
> > +     }
> > +     return -ENOTSUPP;
> > +}
> > +EXPORT_SYMBOL_GPL(l1d_flush_hw);
> > +
> > +void l1d_flush_sw(void *l1d_flush_pages)
> > +{
> > +     int size = PAGE_SIZE << L1D_CACHE_ORDER;
> > +
> > +     asm volatile(
> > +                     /* 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" (l1d_flush_pages),
> > +                     [size] "r" (size)
> > +                     : "eax", "ecx");
> > +}
> 
> If the answer to my previous question in patch 1/6 about the allocation
> being twice the size is yes, then could you allocate the flush pages the
> same size as the cache and just write it twice? Wouldn't that accomplish
> the same goal and provide a performance improvement with (mostly) now
> present L1D entries of the flush pages? Also, can't you unroll this loop a
> bit and operate on multiple entries in each loop, reducing the overall
> looping compares and jumps as an optimization?
> 

Yes, I have some options suggested in my answer to your question in 1/6. For
now I decided to stick with the algorithm already present, but we could
definitely optimize it

Balbir

> Thanks,
> Tom
>