[MODERATED] Re: [PATCH 1/2] L1TF KVM 1

[Jon Masters <jcm@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 1145 bytes --]

On 05/29/2018 06:49 PM, speck for Thomas Gleixner wrote:
> On Tue, 29 May 2018, speck for Paolo Bonzini wrote:

>> +void kvm_l1d_flush(void)
>> +{
>> +	asm volatile(
>> +		"movq %0, %%rax\n\t"
>> +		"leaq 65536(%0), %%rdx\n\t"
> 
> Why 64K?
> 
>> +		"11: \n\t"
>> +		"movzbl (%%rax), %%ecx\n\t"
>> +		"addq $4096, %%rax\n\t"
>> +		"cmpq %%rax, %%rdx\n\t"
>> +		"jne 11b\n\t"
>> +		"xorl %%eax, %%eax\n\t"
>> +		"cpuid\n\t"

My guess is they're saying that the maximum L1D$ size is 64K so they
want to stride it 1 4K page at a time to get the prefetchers going ahead
of the next loop...this in theory will make the following loop "faster".

> What's the cpuid invocation for?
> 
>> +		"xorl %%eax, %%eax\n\t"
>> +		"12:\n\t"
>> +		"movzwl %%ax, %%edx\n\t"
>> +		"addl $64, %%eax\n\t"
>> +		"movzbl (%%rdx, %0), %%ecx\n\t">> +		"cmpl $65536, %%eax\n\t"

...which then tries to do 64 bytes (Intel cache line) at a time.

They use the CPUID as a serializing instruction to ensure the store has
been observed, others have commented on that.

Jon.

-- 
Computer Architect | Sent from my Fedora powered laptop