From: "Luc, Piotr" <Piotr.Luc@intel.com>
To: "bp@alien8.de" <bp@alien8.de>
Cc: "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"he.chen@linux.intel.com" <he.chen@linux.intel.com>,
"tglx@linutronix.de" <tglx@linutronix.de>,
"x86@kernel.org" <x86@kernel.org>,
"hpa@zytor.com" <hpa@zytor.com>,
"mingo@redhat.com" <mingo@redhat.com>,
"pbonzini@redhat.com" <pbonzini@redhat.com>,
"Kang, Luwei" <luwei.kang@intel.com>,
"rkrcmar@redhat.com" <rkrcmar@redhat.com>
Subject: Re: [PATCH] x86/cpuid: expose AVX512_4VNNIW and AVX512_4FMAPS features to kvm guest
Date: Mon, 31 Oct 2016 10:18:41 +0000 [thread overview]
Message-ID: <1477908811.32008.20.camel@intel.com> (raw)
In-Reply-To: <20161031095312.dzv3rj2fqgjjsy7w@pd.tnic>
On Mon, 2016-10-31 at 10:53 +0100, Borislav Petkov wrote:
> > I think that in v4.9-rc2 the CPUID[7,0].edx bits can be masked out
> by
> > applying noxsave to cmdline. Using directly cpu_count will result
> in
> > passing the bits in edx to a guest directly while the xsaveopt and
> rest
> > of AVX512 features bits will be cleared.
>
> Errr, I can't parse that reading it backwards and forwards. Please
> elaborate.
The patch that introduces AVX512_4VNNIW and AVX512_4FMAPS features was
merged to kernel 4.9-rc2 so we have possibility to mask the feature
bits using 'noxsave' option in kernel cmdline. This option clears all
AVX512 feature bits in boot_cpu_data.x86_capability.
The cpuid_mask function, which usually used in kvm, read bit from this
x86_capabity and mask out. This prevents passing disabled features to
guest. If we use cpu_count instead, which reports bits directly from
CPU, then the bits of features that are disabled in host are passed to
guest as enabled. This seems be inconsistent.
Thanks,
Piotr
next prev parent reply other threads:[~2016-10-31 10:18 UTC|newest]
Thread overview: 24+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-10-28 9:12 [PATCH] x86/cpuid: expose AVX512_4VNNIW and AVX512_4FMAPS features to kvm guest He Chen
2016-10-28 9:31 ` Paolo Bonzini
2016-10-28 9:46 ` He Chen
2016-10-28 9:54 ` Paolo Bonzini
2016-10-28 9:55 ` He Chen
2016-10-28 10:13 ` Luc, Piotr
2016-10-28 10:17 ` Paolo Bonzini
2016-10-28 11:08 ` Borislav Petkov
2016-10-28 12:07 ` Paolo Bonzini
2016-10-28 12:21 ` Borislav Petkov
2016-10-29 12:21 ` Paolo Bonzini
2016-10-29 12:25 ` Borislav Petkov
2016-10-29 12:36 ` Paolo Bonzini
2016-10-29 12:53 ` Borislav Petkov
2016-10-29 13:20 ` Paolo Bonzini
2016-10-31 9:15 ` Luc, Piotr
2016-10-31 9:53 ` Borislav Petkov
2016-10-31 10:18 ` Luc, Piotr [this message]
2016-10-31 10:30 ` Borislav Petkov
2016-10-31 10:47 ` Paolo Bonzini
2016-10-31 11:05 ` Borislav Petkov
2016-10-31 11:41 ` Paolo Bonzini
2016-11-01 7:48 ` He Chen
2016-11-01 8:46 ` Borislav Petkov
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