Re: [Xen-devel] [PATCH 2/6] x86emul: support WBNOINVD

From: Jan Beulich <jbeulich@suse.com>
To: Andrew Cooper <andrew.cooper3@citrix.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	Paul Durrant <Paul.Durrant@citrix.com>, Wei Liu <wl@xen.org>,
	RogerPau Monne <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH 2/6] x86emul: support WBNOINVD
Date: Tue, 27 Aug 2019 17:08:55 +0200	[thread overview]
Message-ID: <da32d44d-286b-b80c-f1c0-22ccec9f627a@suse.com> (raw)
In-Reply-To: <06684b85-6500-6066-d282-97ef4c0d1923@citrix.com>

On 27.08.2019 16:47, Andrew Cooper wrote:
> On 01/07/2019 12:56, Jan Beulich wrote:
>> --- a/xen/arch/x86/pv/emul-priv-op.c
>> +++ b/xen/arch/x86/pv/emul-priv-op.c
>> @@ -1121,7 +1121,7 @@ static int write_msr(unsigned int reg, u
>> @@ -1130,6 +1130,8 @@ static int cache_op(enum x86emul_cache_o
>>             * newer linux uses this in some start-of-day timing loops.
>>             */
>>            ;
>> +    else if ( op == x86emul_wbnoinvd && cpu_has_wbnoinvd )
>> +        wbnoinvd();
>>        else
>>            wbinvd();
> 
> The cpu_has_wbnoinvd check isn't necessary.  The encoding was chosen
> because it does get interpreted as wbinvd on older processors.

I agree, but wanted to make the code look complete / consistent.
Would you be okay with the && ... being retained, but in a
comment?

>> --- a/xen/include/asm-x86/system.h
>> +++ b/xen/include/asm-x86/system.h
>> @@ -16,6 +16,11 @@ static inline void wbinvd(void)
>>        asm volatile ( "wbinvd" ::: "memory" );
>>    }
>>    
>> +static inline void wbnoinvd(void)
>> +{
>> +    asm volatile ( "repe; wbinvd" : : : "memory" );
> 
> Semicolon.

It has to stay, as gas rejects use of REP on insns it doesn't think
permit use of REP. H.J. actually proposes even more strict (or should
I say hostile) gas behavior, which would then also reject the above
construct:
https://sourceware.org/ml/binutils/2019-07/msg00186.html

>> --- a/xen/include/public/arch-x86/cpufeatureset.h
>> +++ b/xen/include/public/arch-x86/cpufeatureset.h
>> @@ -243,6 +244,7 @@ XEN_CPUFEATURE(EFRO,          7*32+10) /
>>    
>>    /* AMD-defined CPU features, CPUID level 0x80000008.ebx, word 8 */
>>    XEN_CPUFEATURE(CLZERO,        8*32+ 0) /*A  CLZERO instruction */
>> +XEN_CPUFEATURE(WBNOINVD,      8*32+ 9) /*A  WBNOINVD instruction */
> 
> This is implicitly linked with CPUID.8000001d which we don't expose yet.

On AMD, but not (so far at least, judging from the SDM) on Intel.

> To get the emulation side of things sorted, I'd be happy with this going
> in without the A for now, and "exposing WBNOINVD to guests" can be a
> followup task.

I've dropped the A for now, but as per above I'm not entirely
certain that's appropriate; it's certainly the more defensive step.
My uncertainty is also because people are free to use the WBNOINVD
encoding even without the feature flag set, as it won't #UD (as you
also suggest elsewhere in your reply).

Jan

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xenproject.org
https://lists.xenproject.org/mailman/listinfo/xen-devel