Re: [Xen-devel] [PATCH RFC] x86/HVM: use single (atomic) MOV for aligned emulated writes

[Jan Beulich <jbeulich@xxxxxxxx>]

On 09.09.2019 13:02, Andrew Cooper wrote:
> On 05/09/2019 15:07, Jan Beulich wrote:
>> Using memcpy() may result in multiple individual byte accesses
>> (dependening how memcpy() is implemented and how the resulting insns,
>> e.g. REP MOVSB, get carried out in hardware), which isn't what we
>> want/need for carrying out guest insns as correctly as possible. Fall
>> back to memcpy() only for misaligned accesses as well as ones not 2, 4,
>> or 8 bytes in size.
>>
>> Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> RFC: Besides wanting to hear if this is considered acceptable and
>>      sufficient (or whether it is thought that the linear_write() path
>>      also needs playing with), the question is whether we'd want to
>>      extend this to reads as well. linear_{read,write}() currently don't
>>      use hvmemul_map_linear_addr(), i.e. in both cases I'd need to also
>>      fiddle with __hvm_copy() (perhaps by making the construct below a
>>      helper function).
>>
>> --- a/xen/arch/x86/hvm/emulate.c
>> +++ b/xen/arch/x86/hvm/emulate.c
>> @@ -1352,7 +1352,14 @@ static int hvmemul_write(
>>      if ( !mapping )
>>          return linear_write(addr, bytes, p_data, pfec, hvmemul_ctxt);
>>  
>> -    memcpy(mapping, p_data, bytes);
>> +    /* For aligned accesses use single (and hence atomic) MOV insns. */
>> +    switch ( bytes | ((unsigned long)mapping & (bytes - 1)) )
> 
> Why only for aligned values?  Misaligned are atomic on almost all 64-bit
> capable systems, and mapping will correctly span a page boundary if
> necessary.

Hmm, sure, I can relax this.

>> +    {
>> +    case 2: write_u16_atomic(mapping, *(uint16_t *)p_data); break;
>> +    case 4: write_u32_atomic(mapping, *(uint32_t *)p_data); break;
>> +    case 8: write_u64_atomic(mapping, *(uint64_t *)p_data); break;
>> +    default: memcpy(mapping, p_data, bytes);                break;
>> +    }

Do you have an opinion on whether to have "case 1" here as well, to
avoid going through memcpy()?

Also do you have any opinion on the RFC note further up?

Thanks, Jan

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