Re: [Virtio-fs] [PATCH v4] kvm, x86: Exit to user space in case page fault error

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Vitaly Kuznetsov <vkuznets@redhat.com>,
	virtio-fs-list <virtio-fs@redhat.com>,
	pbonzini@redhat.com, kvm@vger.kernel.org,
	Vivek Goyal <vgoyal@redhat.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [Virtio-fs] [PATCH v4] kvm, x86: Exit to user space in case page fault error
Date: Tue, 6 Oct 2020 18:21:48 +0100	[thread overview]
Message-ID: <20201006172148.GI3000@work-vm> (raw)
In-Reply-To: <20201006171704.GC17610@linux.intel.com>

* Sean Christopherson (sean.j.christopherson@intel.com) wrote:
> On Tue, Oct 06, 2020 at 06:39:56PM +0200, Vitaly Kuznetsov wrote:
> > Sean Christopherson <sean.j.christopherson@intel.com> writes:
> > 
> > > On Tue, Oct 06, 2020 at 05:24:54PM +0200, Vitaly Kuznetsov wrote:
> > >> Vivek Goyal <vgoyal@redhat.com> writes:
> > >> > So you will have to report token (along with -EFAULT) to user space. So this
> > >> > is basically the 3rd proposal which is extension of kvm API and will
> > >> > report say HVA/GFN also to user space along with -EFAULT.
> > >> 
> > >> Right, I meant to say that guest kernel has full register state of the
> > >> userspace process which caused APF to get queued and instead of trying
> > >> to extract it in KVM and pass to userspace in case of a (later) failure
> > >> we limit KVM api change to contain token or GFN only and somehow keep
> > >> the rest in the guest. This should help with TDX/SEV-ES.
> > >
> > > Whatever gets reported to userspace should be identical with and without
> > > async page faults, i.e. it definitely shouldn't have token information.
> > >
> > 
> > Oh, right, when the error gets reported synchronously guest's kernel is
> > not yet aware of the issue so it won't be possible to find anything in
> > its kdump if userspace decides to crash it immediately. The register
> > state (if available) will be actual though.
> > 
> > > Note, TDX doesn't allow injection exceptions, so reflecting a #PF back
> > > into the guest is not an option.  
> > 
> > Not even #MC? So sad :-)
> 
> Heh, #MC isn't allowed either, yet...
> 
> > > Nor do I think that's "correct" behavior (see everyone's objections to
> > > using #PF for APF fixed).  I.e. the event should probably be an IRQ.
> > 
> > I recall Paolo objected against making APF 'page not present' into in
> > interrupt as it will require some very special handling to make sure it
> > gets injected (and handled) immediately but I'm not really sure how big
> > the hack is going to be, maybe in the light of TDX/SEV-ES it's worth a
> > try.
> 
> This shouldn't have anything to do with APF.  Again, the event injection is
> needed even in the synchronous case as the file truncation in the host can
> affect existing mappings in the guest.
> 
> I don't know that the mechanism needs to be virtiofs specific or if there can
> be a more generic "these PFNs have disappeared", but it's most definitely
> orthogonal to APF.

There are other cases we get 'these PFNs have disappeared' other than
virtiofs;  the classic is when people back the guest using a tmpfs that
then runs out of room.

Dave

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-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK