linux-kernel - Re: [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report faults to userspace

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Message-ID: <e6025e28-ea93-458b-887f-925eab7cf260@redhat.com>
Date:   Thu, 2 Nov 2023 12:01:41 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Kai Huang <kai.huang@...el.com>
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        Wei W Wang <wei.w.wang@...el.com>
Subject: Re: [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report
 faults to userspace

On 11/1/23 18:36, Sean Christopherson wrote:
> A good example is KVM_RUN with -EINTR; if KVM were to return something other than
> -EINTR on a pending signal or vcpu->run->immediate_exit, userspace would fall over.

And dually if KVM were to return KVM_EXIT_INTR together with something 
other than -EINTR.

> And purging exit_reason super early is subtly tricky because KVM's 
> (again, poorly documented) ABI is that *some* exit reasons are preserved 
> across KVM_RUN with vcpu->run->immediate_exit (or with a pending 
> signal). https://lore.kernel.org/all/ZFFbwOXZ5uI%2Fgdaf@google.com

vcpu->run->immediate_exit preserves all exit reasons, but it's not a 
good idea that immediate_exit behaves different from a pending signal on 
entry to KVM_RUN (remember that immediate_exit was meant to be a better 
performing alternative to KVM_SET_SIGNAL_MASK).

In principle, vcpu->run->immediate_exit could return KVM_EXIT_INTR 
(perhaps even _should_, except that breaks selftests so at this point it 
is ABI).

Paolo