Re: [PATCH 5/8] KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction

From: "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
To: Sean Christopherson <seanjc@google.com>
Cc: Maxim Levitsky <mlevitsk@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 5/8] KVM: SVM: Re-inject INT3/INTO instead of retrying the instruction
Date: Mon, 4 Apr 2022 21:50:53 +0200	[thread overview]
Message-ID: <53a9ac0a-fce5-69a0-41e5-9a1a2dab9317@maciej.szmigiero.name> (raw)
In-Reply-To: <YktIGHM86jHkzGdF@google.com>

On 4.04.2022 21:33, Sean Christopherson wrote:
> On Mon, Apr 04, 2022, Maciej S. Szmigiero wrote:
>>>>> index 47e7427d0395..a770a1c7ddd2 100644
>>>>> --- a/arch/x86/kvm/svm/svm.h
>>>>> +++ b/arch/x86/kvm/svm/svm.h
>>>>> @@ -230,8 +230,8 @@ struct vcpu_svm {
>>>>>    	bool nmi_singlestep;
>>>>>    	u64 nmi_singlestep_guest_rflags;
>>>>> -	unsigned int3_injected;
>>>>> -	unsigned long int3_rip;
>>>>> +	unsigned soft_int_injected;
>>>>> +	unsigned long soft_int_linear_rip;
>>>>>    	/* optional nested SVM features that are enabled for this guest  */
>>>>>    	bool nrips_enabled                : 1;
>>>>
>>>>
>>>> I mostly agree with this patch, but think that it doesn't address the
>>>> original issue that Maciej wanted to address:
>>>>
>>>> Suppose that there is *no* instruction in L2 code which caused the software
>>>> exception, but rather L1 set arbitrary next_rip, and set EVENTINJ to software
>>>> exception with some vector, and that injection got interrupted.
>>>>
>>>> I don't think that this code will support this.
>>>
>>> Argh, you're right.  Maciej's selftest injects without an instruction, but it doesn't
>>> configure the scenario where that injection fails due to an exception+VM-Exit that
>>> isn't intercepted by L1 and is handled by L0.  The event_inj test gets the coverage
>>> for the latter, but always has a backing instruction.
>>
>> Still reviewing the whole patch set, but want to clear this point quickly:
>> The selftest does have an implicit intervening NPF (handled by L0) while
>> injecting the first L1 -> L2 event.
> 
> I'll do some debug to figure out why the test passes for me.  I'm guessing I either
> got lucky, e.g. IDT was faulted in already, or I screwed up and the test doesn't
> actually pass.

Try applying Maxim's proposed change to this test (adding two additional ud2s at the
start of L2 guest code and adjusting just the next_rip field accordingly).

We might have been lucky in the patched KVM code and have next_rip = rip at this
re-injection case now (the next_rip field was zero in the original KVM code).

Thanks,
Maciej