linux-kernel - Re: [PATCH 5/5] KVM: vmx: use MSR_IA32_TSX_CTRL to hard-disable TSX on guest that lack it

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Message-ID: <4a820b71-79eb-7d71-9cd2-93954cefcdd2@redhat.com>
Date:   Thu, 21 Nov 2019 10:05:04 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Eduardo Habkost <ehabkost@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        jmattson@...gle.com,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH 5/5] KVM: vmx: use MSR_IA32_TSX_CTRL to hard-disable TSX
 on guest that lack it

On 21/11/19 03:22, Eduardo Habkost wrote:
> On Mon, Nov 18, 2019 at 07:17:47PM +0100, Paolo Bonzini wrote:
>> If X86_FEATURE_RTM is disabled, the guest should not be able to access
>> MSR_IA32_TSX_CTRL.  We can therefore use it in KVM to force all
>> transactions from the guest to abort.
>>
>> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> 
> So, without this patch guest OSes will incorrectly report "Not
> affected" at /sys/devices/system/cpu/vulnerabilities/tsx_async_abort
> if RTM is disabled in the VM configuration.
> 
> Is there anything host userspace can do to detect this situation
> and issue a warning on that case?
> 
> Is there anything the guest kernel can do to detect this and not
> report a false negative at /sys/.../tsx_async_abort?

Unfortunately not.  The hypervisor needs to know about TAA in order to
mitigate it on behalf of the guest.  At least this doesn't require an
updated userspace and VM configuration!

Paolo