linux-kernel - Re: [PATCH v2 03/16] KVM: x86: Always use non-compat vcpu_runstate

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Message-ID: <24768aa3-0e2e-6d29-2749-9d74a26f9205@redhat.com>
Date:   Thu, 27 Oct 2022 17:03:08 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Michal Luczaj <mhal@...x.co>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [PATCH v2 03/16] KVM: x86: Always use non-compat
 vcpu_runstate_info size for gfn=>pfn cache

On 10/27/22 16:44, Sean Christopherson wrote:
>> - long mode cannot be changed after the shared info page is enabled (which
>> makes sense because the shared info page also has a compat version)
> 
> How is this not introducing an additional restriction?  This seems way more
> onerous than what is effectively a revert.
>
>> - the caches must be activated after the shared info page (which enforces
>> that the vCPU attributes are set after the VM attributes)
>>
>> This is technically a userspace API break, but nobody is really using this
>> API outside Amazon so...  Patches coming after I finish testing.
> 
> It's not just userspace break, it affects the guest ABI as well.

Yes, I was talking of the VMM here; additional restrictions are fine there.

The guests however should be compatible with Xen, so you also need to 
re-activate the cache after the hypercall page is written, but that's 
two lines of code.

Paolo