linux-kernel - Re: KVM_SET_CPUID doesn't check supported bits (was Re: [PATCH 0/6] KVM: x86: KVM_SET

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Date:   Mon, 7 Dec 2020 17:03:55 +0300
From:   stsp <stsp2@...dex.ru>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: KVM_SET_CPUID doesn't check supported bits (was Re: [PATCH 0/6]
 KVM: x86: KVM_SET_SREGS.CR4 bug fixes and cleanup)

07.12.2020 16:35, Paolo Bonzini пишет:
>
>
> Il lun 7 dic 2020, 12:47 stsp <stsp2@...dex.ru 
> <mailto:stsp2@...dex.ru>> ha scritto:
>
>     So am I right that KVM_SET_CPUID only "lowers"
>     the supported bits? In which case I don't need to
>     call it at all, but instead just call KVM_GET_SUPPORTED_CPUID
>     and see if the needed bits are supported, and
>     exit otherwise, right?
>
>
> You always have to call KVM_SET_CPUID2, but you can just pass in 
> whatever you got from KVM_GET_SUPPORTED_CPUID.
OK, done that, thanks.
(after checking that KVM_GET_SUPPORTED_CPUID
actually has the needed features itself, otherwise exit).

Perhaps it would be good if guest cpuid to
have a default values of KVM_GET_SUPPORTED_CPUID,
so that the user doesn't have to do the needless
calls to just copy host features to guest cpuid.