Re: [Xen-devel] [PATCH 3/9] x86/mm: honor opt_pcid also for 32-bit PV domains

[Jan Beulich <jbeulich@suse.com>]

On 12.09.2019 12:34, Roger Pau Monné  wrote:
> On Wed, Sep 11, 2019 at 05:22:51PM +0200, Jan Beulich wrote:
>> I can't see any technical or performance reason why we should treat
>> 32-bit PV different from 64-bit PV in this regard.
>>
>> Signed-off-by: Jan Beulich <jbeulich@suse.com>
> 
> The original commit mentions that PCID doesn't improve performance for
> non-XPTI domains, but it doesn't mention whether it makes performance
> worse.

Well, yes - it's not like we're defaulting to using PCID now for
32-bit guests. But we allow people to turn on its use. After all
the original measurements were done on a limited set of hardware,
and hardware also changes/advances all the time.

> The change LGTM, if you are fine performance wise:
> 
> Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>

Thanks.

>> --- a/xen/arch/x86/pv/domain.c
>> +++ b/xen/arch/x86/pv/domain.c
>> @@ -180,7 +180,24 @@ int switch_compat(struct domain *d)
>>      d->arch.x87_fip_width = 4;
>>  
>>      d->arch.pv.xpti = false;
>> -    d->arch.pv.pcid = false;
>> +
>> +    if ( use_invpcid && cpu_has_pcid )
>> +        switch ( ACCESS_ONCE(opt_pcid) )
>> +        {
>> +        case PCID_OFF:
>> +        case PCID_XPTI:
>> +            d->arch.pv.pcid = false;
>> +            break;
>> +
>> +        case PCID_ALL:
>> +        case PCID_NOXPTI:
>> +            d->arch.pv.pcid = true;
>> +            break;
>> +
>> +        default:
>> +            ASSERT_UNREACHABLE();
>> +            break;
>> +        }
> 
> This chunk is (functionality wise) exactly the same as the one in
> pv_domain_initialise, it might be good to put this in a separate
> helper?

Could be, indeed, but would at least double the size of this patch.
I wasn't convinced that's worth it. I'll see what Andrew thinks,
since I'll need his ack anyway (at least in my understanding of the
still un-refined, un-written rules of what is necessary for
committing a maintainer's patch).

Jan

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