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 15:59:19 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     stsp <stsp2@...dex.ru>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <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)

On Mon, Dec 7, 2020 at 3:47 AM stsp <stsp2@...dex.ru> wrote:
>
> 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?

"Lowers" is a tricky concept for CPUID information. Some feature bits
report 0 for "present" and 1 for "not-present." Some multi-bit fields
are interpreted as numbers, which may be signed or unsigned. Some
multi-bit fields are strings. Some fields have dependencies on other
fields. Etc.