Re: [PATCH v2] KVM: X86: Fix exception untrigger on ret to user

From: stsp <stsp2@yandex.ru>
To: Jim Mattson <jmattson@google.com>
Cc: Maxim Levitsky <mlevitsk@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Joerg Roedel <joro@8bytes.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Jan Kiszka <jan.kiszka@siemens.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	kvm@vger.kernel.org
Subject: Re: [PATCH v2] KVM: X86: Fix exception untrigger on ret to user
Date: Wed, 7 Jul 2021 19:34:55 +0300	[thread overview]
Message-ID: <6b263403-f959-6d9a-2bbe-15a684df6f50@yandex.ru> (raw)
In-Reply-To: <CALMp9eQo6aUCz6+KOWJLhOXJET+4HHVA-HyhjHzAjnfFgTec4Q@mail.gmail.com>

07.07.2021 19:16, Jim Mattson пишет:
> On Tue, Jul 6, 2021 at 4:06 PM stsp <stsp2@yandex.ru> wrote:
>> 07.07.2021 02:00, Maxim Levitsky пишет:
>>> On Wed, 2021-07-07 at 00:50 +0300, stsp wrote:
>>>> 06.07.2021 23:29, Maxim Levitsky пишет:
>>>>> On Tue, 2021-07-06 at 15:06 +0300, stsp wrote:
>>>>>> 06.07.2021 14:49, Maxim Levitsky пишет:
>>>>>>> Now about the KVM's userspace API where this is exposed:
>>>>>>>
>>>>>>> I see now too that KVM_SET_REGS clears the pending exception.
>>>>>>> This is new to me and it is IMHO *wrong* thing to do.
>>>>>>> However I bet that someone somewhere depends on this,
>>>>>>> since this behavior is very old.
>>>>>> What alternative would you suggest?
>>>>>> Check for ready_for_interrupt_injection
>>>>>> and never call KVM_SET_REGS if it indicates
>>>>>> "not ready"?
>>>>>> But what if someone calls it nevertheless?
>>>>>> Perhaps return an error from KVM_SET_REGS
>>>>>> if exception is pending? Also KVM_SET_SREGS
>>>>>> needs some treatment here too, as it can
>>>>>> also be called when an exception is pending,
>>>>>> leading to problems.
>>>>> As I explained you can call KVM_GET_VCPU_EVENTS before calling
>>>>> KVM_SET_REGS and then call KVM_SET_VCPU_EVENTS with the struct
>>>>> that was filled by KVM_GET_VCPU_EVENTS.
>>>>> That will preserve all the cpu events.
>>>> The question is different.
>>>> I wonder how _should_ the KVM
>>>> API behave when someone calls
>>>> KVM_SET_REGS/KVM_SET_SREGS
>>> KVM_SET_REGS should not clear the pending exception.
>>> but fixing this can break API compatibilitly if some
>>> hypervisor (not qemu) relies on it.
>>>
>>> Thus either a new ioctl is needed or as I said,
>>> KVM_GET_VCPU_EVENTS/KVM_SET_VCPU_EVENTS can be used
>>> to preserve the events around that call as workaround.
>> But I don't need to preserve
>> events. Canceling is perfectly
>> fine with me because, if I inject
>> the interrupt at that point, the
>> exception will be re-triggered
>> anyway after interrupt handler
>> returns.
> The exception will not be re-triggered if it was a trap,
But my assumption was that
everything is atomic, except
PF with shadow page tables.
I guess you mean the cases
when the exception delivery
causes EPT fault, which is a bit
of a corner case.