From: Paolo Bonzini <pbonzini@redhat.com>
To: "Tian, Kevin" <kevin.tian@intel.com>, "Christopherson,,
Sean" <seanjc@google.com>, Like Xu <like.xu.linux@gmail.com>
Cc: "Liu, Jing2" <jing2.liu@intel.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>,
Vitaly Kuznetsov <vkuznets@redhat.com>,
Joerg Roedel <joro@8bytes.org>,
"kvm@vger.kernel.org" <kvm@vger.kernel.org>,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
"Zhong, Yang" <yang.zhong@intel.com>
Subject: Re: [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0
Date: Tue, 25 Jan 2022 09:27:47 +0100 [thread overview]
Message-ID: <f00d0e56-e5d3-4ac6-1519-fa843fb4d734@redhat.com> (raw)
In-Reply-To: <BN9PR11MB5276170712A9EF842B36ACE48C5F9@BN9PR11MB5276.namprd11.prod.outlook.com>
On 1/25/22 02:54, Tian, Kevin wrote:
>> The extra complication is that arch_prctl(ARCH_REQ_XCOMP_GUEST_PERM)
>> changes what host userspace can/can't do. It would be easier if we
>> could just say that KVM_GET_SUPPORTED_CPUID returns "the most" that
>> userspace can do, but we already have the contract that userspace can
>> take KVM_GET_SUPPORTED_CPUID and pass it straight to KVM_SET_CPUID2.
>>
>> Therefore, KVM_GET_SUPPORTED_CPUID must limit its returned values to
>> what has already been enabled.
>>
>> While reviewing the QEMU part of AMX support (this morning), I also
>> noticed that there is no equivalent for guest permissions of
>> ARCH_GET_XCOMP_SUPP. This needs to know KVM's supported_xcr0, so it's
>> probably best realized as a new KVM_CHECK_EXTENSION rather than as an
>> arch_prctl.
>>
> Would that lead to a weird situation where although KVM says no support
> of guest permissions while the user can still request them via prctl()?
This is already the case for the current implementation of
KVM_GET_SUPPORTED_CPUID.
Paolo
> I wonder whether it's cleaner to do it still via prctl() if we really want to
> enhance this part. But as you said then it needs a mechanism to know
> KVM's supported_xcr0 (and if KVM is not loaded then no guest permission
> support at all)...
next prev parent reply other threads:[~2022-01-25 8:30 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-01-23 5:50 [PATCH] KVM: x86/cpuid: Exclude unpermitted xfeatures for vcpu->arch.guest_supported_xcr0 Like Xu
2022-01-24 7:06 ` Tian, Kevin
2022-01-24 7:17 ` Like Xu
2022-01-24 16:23 ` Sean Christopherson
2022-01-24 16:29 ` Paolo Bonzini
2022-01-25 1:54 ` Tian, Kevin
2022-01-25 8:27 ` Paolo Bonzini [this message]
2022-01-26 1:23 ` Tian, Kevin
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