linux-kernel - Re: [PATCH 3/5] KVM: nSVM: Don't forget about L1-injected events

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Message-ID: <8f9ae64a-dc64-6f46-8cd4-ffd2648a9372@maciej.szmigiero.name>
Date:   Thu, 31 Mar 2022 00:16:59 +0200
From:   "Maciej S. Szmigiero" <mail@...iej.szmigiero.name>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Brijesh Singh <brijesh.singh@....com>,
        Jon Grimm <Jon.Grimm@....com>,
        David Kaplan <David.Kaplan@....com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Liam Merwick <liam.merwick@...cle.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/5] KVM: nSVM: Don't forget about L1-injected events

On 30.03.2022 23:59, Sean Christopherson wrote:
> On Thu, Mar 10, 2022, Maciej S. Szmigiero wrote:
>> From: "Maciej S. Szmigiero" <maciej.szmigiero@...cle.com>
>>
>> In SVM synthetic software interrupts or INT3 or INTO exception that L1
>> wants to inject into its L2 guest are forgotten if there is an intervening
>> L0 VMEXIT during their delivery.
>>
>> They are re-injected correctly with VMX, however.
>>
>> This is because there is an assumption in SVM that such exceptions will be
>> re-delivered by simply re-executing the current instruction.
>> Which might not be true if this is a synthetic exception injected by L1,
>> since in this case the re-executed instruction will be one already in L2,
>> not the VMRUN instruction in L1 that attempted the injection.
>>
>> Leave the pending L1 -> L2 event in svm->nested.ctl.event_inj{,err} until
>> it is either re-injected successfully or returned to L1 upon a nested
>> VMEXIT.
>> Make sure to always re-queue such event if returned in EXITINTINFO.
>>
>> The handling of L0 -> {L1, L2} event re-injection is left as-is to avoid
>> unforeseen regressions.
>>
>> Signed-off-by: Maciej S. Szmigiero <maciej.szmigiero@...cle.com>
>> ---
> 
> ...
> 
>> @@ -3627,6 +3632,14 @@ static void svm_complete_interrupts(struct kvm_vcpu *vcpu)
>>   	if (!(exitintinfo & SVM_EXITINTINFO_VALID))
>>   		return;
>>   
>> +	/* L1 -> L2 event re-injection needs a different handling */
>> +	if (is_guest_mode(vcpu) &&
>> +	    exit_during_event_injection(svm, svm->nested.ctl.event_inj,
>> +					svm->nested.ctl.event_inj_err)) {
>> +		nested_svm_maybe_reinject(vcpu);
> 
> Why is this manually re-injecting?  More specifically, why does the below (out of
> sight in the diff) code that re-queues the exception/interrupt not work?  The
> re-queued event should be picked up by nested_save_pending_event_to_vmcb12() and
> propagatred to vmcb12.

A L1 -> L2 injected event should either be re-injected until successfully
injected into L2 or propagated to VMCB12 if there is a nested VMEXIT
during its delivery.

svm_complete_interrupts() does not do such re-injection in some cases
(soft interrupts, soft exceptions, #VC) - it is trying to resort to
emulation instead, which is incorrect in this case.

I think it's better to split out this L1 -> L2 nested case to a
separate function in nested.c rather than to fill
svm_complete_interrupts() in already very large svm.c with "if" blocks
here and there.

Thanks,
Maciej