From: Thomas Gleixner <tglx@linutronix.de>
To: Vivek Goyal <vgoyal@redhat.com>, Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>,
Paolo Bonzini <pbonzini@redhat.com>,
LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
kvm list <kvm@vger.kernel.org>, stable <stable@vger.kernel.org>
Subject: Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS
Date: Mon, 06 Apr 2020 23:32:31 +0200 [thread overview]
Message-ID: <87wo6sjo5s.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20200406190951.GA19259@redhat.com>
Vivek Goyal <vgoyal@redhat.com> writes:
> On Mon, Mar 09, 2020 at 09:22:15PM +0100, Peter Zijlstra wrote:
>> On Mon, Mar 09, 2020 at 08:05:18PM +0100, Thomas Gleixner wrote:
>> > Andy Lutomirski <luto@kernel.org> writes:
>>
>> > > I'm okay with the save/restore dance, I guess. It's just yet more
>> > > entry crud to deal with architecture nastiness, except that this
>> > > nastiness is 100% software and isn't Intel/AMD's fault.
>> >
>> > And we can do it in C and don't have to fiddle with it in the ASM
>> > maze.
>>
>> Right; I'd still love to kill KVM_ASYNC_PF_SEND_ALWAYS though, even if
>> we do the save/restore in do_nmi(). That is some wild brain melt. Also,
>> AFAIK none of the distros are actually shipping a PREEMPT=y kernel
>> anyway, so killing it shouldn't matter much.
>
> It will be nice if we can retain KVM_ASYNC_PF_SEND_ALWAYS. I have another
> use case outside CONFIG_PREEMPT.
>
> I am trying to extend async pf interface to also report page fault errors
> to the guest.
>
> https://lore.kernel.org/kvm/20200331194011.24834-1-vgoyal@redhat.com/
>
> Right now async page fault interface assumes that host will always be
> able to successfully resolve the page fault and sooner or later PAGE_READY
> event will be sent to guest. And there is no mechnaism to report the
> errors back to guest.
>
> I am trying to add enhance virtiofs to directly map host page cache in guest.
>
> https://lore.kernel.org/linux-fsdevel/20200304165845.3081-1-vgoyal@redhat.com/
>
> There it is possible that a file page on host is mapped in guest and file
> got truncated and page is not there anymore. Guest tries to access it,
> and it generates async page fault. On host we will get -EFAULT and I
> need to propagate it back to guest so that guest can either send SIGBUS
> to process which caused this. Or if kernel was trying to do memcpy(),
> then be able to use execpetion table error handling and be able to
> return with error. (memcpy_mcflush()).
>
> For the second case to work, I will need async pf events to come in
> even if guest is in kernel and CONFIG_PREEMPT=n.
What?
> So it would be nice if we can keep KVM_ASYNC_PF_SEND_ALWAYS around.
No. If you want this stuff to be actually useful and correct, then
please redesign it from scratch w/o abusing #PF. It want's to be a
separate vector and then the pagefault resulting from your example above
becomes a real #PF without any bells and whistels.
Thanks,
tglx
prev parent reply other threads:[~2020-04-06 21:32 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-07 2:26 [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS Andy Lutomirski
2020-03-07 15:03 ` Andy Lutomirski
2020-03-07 15:47 ` Thomas Gleixner
2020-03-07 15:59 ` Andy Lutomirski
2020-03-07 19:01 ` Thomas Gleixner
2020-03-07 19:34 ` Andy Lutomirski
2020-03-08 7:23 ` Thomas Gleixner
2020-03-09 6:57 ` Thomas Gleixner
2020-03-09 8:40 ` Paolo Bonzini
2020-03-09 9:09 ` Thomas Gleixner
2020-03-09 18:14 ` Andy Lutomirski
2020-03-09 19:05 ` Thomas Gleixner
2020-03-09 20:22 ` Peter Zijlstra
2020-04-06 19:09 ` Vivek Goyal
2020-04-06 20:25 ` Peter Zijlstra
2020-04-06 20:32 ` Andy Lutomirski
2020-04-06 20:42 ` Andy Lutomirski
2020-04-07 17:21 ` Vivek Goyal
2020-04-07 17:38 ` Andy Lutomirski
2020-04-07 20:20 ` Thomas Gleixner
2020-04-07 21:41 ` Andy Lutomirski
2020-04-07 22:07 ` Paolo Bonzini
2020-04-07 22:29 ` Andy Lutomirski
2020-04-08 0:30 ` Paolo Bonzini
2020-05-21 15:55 ` Vivek Goyal
2020-04-07 22:48 ` Thomas Gleixner
2020-04-08 4:48 ` Andy Lutomirski
2020-04-08 9:32 ` Borislav Petkov
2020-04-08 10:12 ` Thomas Gleixner
2020-04-08 18:23 ` Vivek Goyal
2020-04-07 22:49 ` Vivek Goyal
2020-04-08 10:01 ` Borislav Petkov
2020-04-07 22:04 ` Paolo Bonzini
2020-04-07 23:21 ` Thomas Gleixner
2020-04-08 8:23 ` Paolo Bonzini
2020-04-08 13:01 ` Thomas Gleixner
2020-04-08 15:38 ` Peter Zijlstra
2020-04-08 16:41 ` Thomas Gleixner
2020-04-09 9:03 ` Paolo Bonzini
2020-04-08 15:34 ` Sean Christopherson
2020-04-08 16:50 ` Paolo Bonzini
2020-04-08 18:01 ` Thomas Gleixner
2020-04-08 20:34 ` Vivek Goyal
2020-04-08 23:06 ` Thomas Gleixner
2020-04-08 23:14 ` Thomas Gleixner
2020-04-09 4:50 ` Andy Lutomirski
2020-04-09 9:43 ` Paolo Bonzini
2020-04-09 11:36 ` Andrew Cooper
2020-04-09 12:47 ` Paolo Bonzini
2020-04-09 14:13 ` Andrew Cooper
2020-04-09 14:32 ` Paolo Bonzini
2020-04-09 15:03 ` Andy Lutomirski
2020-04-09 15:17 ` Paolo Bonzini
2020-04-09 17:32 ` Andy Lutomirski
2020-04-06 21:32 ` Thomas Gleixner [this message]
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