Re: [Xen-devel] [PATCH] x86/tlb: Opencode the use of write_cr4() in write_cr3() and flush_area_local()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 21/08/17 13:46, Jan Beulich wrote:
>>>> On 21.08.17 at 13:55, <andrew.cooper3@xxxxxxxxxx> wrote:
>> This avoids unnecessary updates to the stack shadow copy of cr4 during
>> critical regions with interrupts disabled.
> Hmm, yes - we don't access CR4 in #MC or NMI handling, do we?

#MC and NMI are always able to observe stale values whether we update
the shadow copy here or not.

The entry paths do modify cr4 to re-enable cr4_pv32_mask, but that is
all.  As we are interrupting Xen context here, we should take the
fastpath and leave cr4 unmodified.

~Andrew

>
>> No change in behaviour.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> with ...
>
>> --- a/xen/arch/x86/flushtlb.c
>> +++ b/xen/arch/x86/flushtlb.c
>> @@ -81,9 +81,14 @@ void write_cr3(unsigned long cr3)
>>  
>>      hvm_flush_guest_tlbs();
>>  
>> -    write_cr4(cr4 & ~X86_CR4_PGE);
>> -    asm volatile ( "mov %0, %%cr3" : : "r" (cr3) : "memory" );
>> -    write_cr4(cr4);
>> +    asm volatile ("mov %[npge], %%cr4;"
>> +                  "mov %[cr3], %%cr3;"
>> +                  "mov %[cr4], %%cr4;"
>> +                  ::
>> +                   [npge] "r" (cr4 & ~X86_CR4_PGE),
>> +                   [cr3]  "r" (cr3),
>> +                   [cr4]  "r" (cr4)
>> +                  : "memory");
> ... blanks added immediately inside the parentheses here and ...
>
>> @@ -123,9 +128,11 @@ unsigned int flush_area_local(const void *va, unsigned 
>> int flags)
>>  
>>              hvm_flush_guest_tlbs();
>>  
>> -            write_cr4(cr4 & ~X86_CR4_PGE);
>> -            barrier();
>> -            write_cr4(cr4);
>> +            asm volatile ("mov %[npge], %%cr4;"
>> +                          "mov %[cr4], %%cr4;"
>> +                          ::
>> +                           [npge] "r" (cr4 & ~X86_CR4_PGE),
>> +                           [cr4]  "r" (cr4));
> ... here.
>
> Jan
>


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