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 14:47:28 +0300
From:   stsp <stsp2@...dex.ru>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     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 14:29, Paolo Bonzini пишет:
> On 07/12/20 12:24, stsp wrote:
>> It tries to enable VME among other things.
>> qemu appears to disable VME by default,
>> unless you do "-cpu host". So we have a situation where
>> the host (which is qemu) doesn't have VME,
>> and guest (dosemu) is trying to enable it.
>> Now obviously KVM_SET_CPUID doesn't check anyting
>> at all and returns success. That later turns
>> into an invalid guest state.
>>
>>
>> Question: should KVM_SET_CPUID check for
>> supported bits, end return error if not everything
>> is supported?
>
> No, it is intentional.  Most bits of CPUID are not ever checked by 
> KVM, so userspace is supposed to set values that makes sense
By "that makes sense" you probably
meant to say "bits_that_makes_sense masked
with the ones returned by KVM_GET_SUPPORTED_CPUID"?

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?