Re: [PATCH v3 2/5] arch/x86: Refactor tlbflush and l1d flush
From: Singh, Balbir
Date: Fri Apr 17 2020 - 18:59:24 EST
On Fri, 2020-04-17 at 15:03 +0200, Thomas Gleixner wrote:
>
> Balbir Singh <sblbir@xxxxxxxxxx> 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