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Message-ID: <24768aa3-0e2e-6d29-2749-9d74a26f9205@redhat.com>
Date:   Thu, 27 Oct 2022 17:03:08 +0200
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Subject: Re: [PATCH v2 03/16] KVM: x86: Always use non-compat
 vcpu_runstate_info size for gfn=>pfn cache
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To:     Sean Christopherson <seanjc@google.com>
Cc:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        Michal Luczaj <mhal@rbox.co>,
        David Woodhouse <dwmw2@infradead.org>
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 <afad5f40-03ef-1380-9bfe-03bbaaed47a9@redhat.com>
 <Y1qZagwM0dMBjYhe@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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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