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

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Date:   Fri, 17 Apr 2020 15:03:26 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Balbir Singh <sblbir@...zon.com>, linux-kernel@...r.kernel.org
Cc:     jpoimboe@...hat.com, tony.luck@...el.com, keescook@...omium.org,
        benh@...nel.crashing.org, x86@...nel.org, dave.hansen@...el.com,
        Balbir Singh <sblbir@...zon.com>
Subject: Re: [PATCH v3 2/5] arch/x86: Refactor tlbflush and l1d flush

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?

> +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.

> +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.

> -	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.

Thanks,

        tglx