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

From: stsp <stsp2@yandex.ru>
To: Maxim Levitsky <mlevitsk@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Sean Christopherson <seanjc@google.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Jim Mattson <jmattson@google.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 02:06:01 +0300	[thread overview]
Message-ID: <dc6a64d0-a77e-b85e-6a8e-c5ca3b42dd59@yandex.ru> (raw)
In-Reply-To: <6c0a0ffe6103272b648dbc3099f0445d5458059b.camel@redhat.com>

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.

What I ask is how SHOULD the
KVM_SET_REGS and KVM_SET_SREGS
behave when someone (mistakenly)
calls them with the exception pending.
Should they return an error
instead of canceling exception?