On Tue, Oct 06, 2020 at 06:21:48PM +0100, Dr. David Alan Gilbert wrote: > * Sean Christopherson (sean.j.christopherson@xxxxxxxxx) wrote: > > On Tue, Oct 06, 2020 at 06:39:56PM +0200, Vitaly Kuznetsov wrote: > > > Sean Christopherson <sean.j.christopherson@xxxxxxxxx> writes: > > > > > > > On Tue, Oct 06, 2020 at 05:24:54PM +0200, Vitaly Kuznetsov wrote: > > > >> Vivek Goyal <vgoyal@xxxxxxxxxx> 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. I also played with nvdimm driver where device was backed a file on host. If I truncate that file, we face similar issues. https://lore.kernel.org/kvm/20200616214847.24482-1-vgoyal@xxxxxxxxxx/ I think any resource which can be backed by a file on host, can potentially run into this issue if file is truncated. (if guest can do load/store on these pages directly). Thanks Vivek
