Re: [PATCH v2 0/2] Dirtying, failing memop: don't indicate suppression

From: Janosch Frank <frankja@linux.ibm.com>
To: Janis Schoetterl-Glausch <scgl@linux.ibm.com>,
	Christian Borntraeger <borntraeger@linux.ibm.com>,
	Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: David Hildenbrand <david@redhat.com>,
	Heiko Carstens <hca@linux.ibm.com>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Sven Schnelle <svens@linux.ibm.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	kvm@vger.kernel.org, Shuah Khan <shuah@kernel.org>,
	linux-kselftest@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-s390@vger.kernel.org
Subject: Re: [PATCH v2 0/2] Dirtying, failing memop: don't indicate suppression
Date: Tue, 26 Apr 2022 14:34:53 +0200	[thread overview]
Message-ID: <b6727a68-5918-c13d-d75c-2ea3f1a2469d@linux.ibm.com> (raw)
In-Reply-To: <80ca4468-6b0a-e8d9-9922-1fae2a1f0fcc@linux.ibm.com>

[...]
>>>
>>> The only question is, do we need to change the suppression parameter in
>>> access_guest_with_key
>>>
>>>     (mode != GACC_STORE) || (idx == 0)
>>>
>>> to also check for prot != PROT_TYPE_KEYC
>>> ? I think we do not need this as we have checked other reasons before.
> 
> Yes, it is not necessary, the control flow is such that a protection exception
> implies that is due to keys.
>>
>> To me this measure looks like a last resort option and the POP doesn't state a 100% what is to be done. Some instructions can mandate suppression instead of termination according to the architects.
>>
>> My intuition tells me that if we are in a situation where this would happen then we would be much better off just doing it by hand (i.e. in the instruction emulation code) and not letting this function decide.
> 
> For the instructions we currently need to emulate in KVM we should be fine.
> So the question is what's best for the future and for instructions emulated by user space.
> Upward in the call stack (including user space), we don't know the failing address,
> which complicates handling it in the emulation code.
> You could chop up the memop in page chunks to find out, but that might have other issues.
> 
> Since this behavior is very implicit and easy to overlook maybe we should document it
> in the description of the memop ioctl?

Yeah, properly documenting this is the least we can do.