linux-kernel - Re: [PATCH v2 11/12] KVM: selftests / xen: don't explicitly set the vcpu

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Date:   Mon, 18 Sep 2023 15:14:50 +0100
From:   Paul Durrant <xadimgnik@...il.com>
To:     David Woodhouse <dwmw2@...radead.org>,
        Paul Durrant <xadimgnik@...il.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     Paul Durrant <pdurrant@...zon.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2 11/12] KVM: selftests / xen: don't explicitly set the
 vcpu_info address

On 18/09/2023 15:05, David Woodhouse wrote:
> 
> 
> On 18 September 2023 14:41:08 BST, Paul Durrant <xadimgnik@...il.com> wrote:
>> Well, if the VMM is using the default then it can't unmap it. But setting a vcpu_info *after* enabling any event channels would be a very odd thing for a guest to do and IMO it gets to keep the pieces if it does so.
> 
> 
> Hm, I suppose I'm OK with that approach. The fact that both VMM implementations using this KVM/Xen support let the guest keep precisely those pieces is a testament to that :)
> 

I can have the selftest explicitly set the vcpu_info to point at the one 
that's already in use, I suppose... so the would at least make sure the 
attribute is functioning.

> But now we're hard-coding the behaviour in the kernel and declaring that no VMM will be *able* to "fix" that case even if it does want to. So perhaps it wants a modicum more thought and at least some explicit documentation to that effect?
> 
> And a hand-wavy plan at least for what we'd do if we suddenly did find a reason to care?

Handwavy plan would be for the VMM to:

a) Mask all open event channels targetting the vcpu
b) Copy vcpu_info content to the new location
c) Tell KVM where it is
d) Unmask the masked event channels

Does that sound ok? If so I can stick it in the API documentation.