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Date:   Thu, 2 Nov 2023 12:01:41 +0100
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Subject: Re: [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report
 faults to userspace
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To:     Sean Christopherson <seanjc@google.com>,
        Kai Huang <kai.huang@intel.com>
Cc:     "viro@zeniv.linux.org.uk" <viro@zeniv.linux.org.uk>,
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        Yilun Xu <yilun.xu@intel.com>,
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References: <20231027182217.3615211-1-seanjc@google.com>
 <20231027182217.3615211-10-seanjc@google.com>
 <482bfea6f54ea1bb7d1ad75e03541d0ba0e5be6f.camel@intel.com>
 <ZUKMsOdg3N9wmEzy@google.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
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In-Reply-To: <ZUKMsOdg3N9wmEzy@google.com>
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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