Date: Tue, 6 Oct 2020 18:21:48 +0100 From: "Dr. David Alan Gilbert" Message-ID: <20201006172148.GI3000@work-vm> References: <20201005161620.GC11938@linux.intel.com> <20201006134629.GB5306@redhat.com> <877ds38n6r.fsf@vitty.brq.redhat.com> <20201006141501.GC5306@redhat.com> <874kn78l2z.fsf@vitty.brq.redhat.com> <20201006150817.GD5306@redhat.com> <871rib8ji1.fsf@vitty.brq.redhat.com> <20201006161200.GB17610@linux.intel.com> <87y2kj71gj.fsf@vitty.brq.redhat.com> <20201006171704.GC17610@linux.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20201006171704.GC17610@linux.intel.com> Subject: Re: [Virtio-fs] [PATCH v4] kvm, x86: Exit to user space in case page fault error List-Id: Development discussions about virtio-fs List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Sean Christopherson Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, virtio-fs-list , pbonzini@redhat.com, Vitaly Kuznetsov , Vivek Goyal * Sean Christopherson (sean.j.christopherson@intel.com) wrote: > On Tue, Oct 06, 2020 at 06:39:56PM +0200, Vitaly Kuznetsov wrote: > > Sean Christopherson writes: > > > > > On Tue, Oct 06, 2020 at 05:24:54PM +0200, Vitaly Kuznetsov wrote: > > >> Vivek Goyal 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 > _______________________________________________ > Virtio-fs mailing list > Virtio-fs@redhat.com > https://www.redhat.com/mailman/listinfo/virtio-fs -- Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK