Messages in this thread | | | Date | Mon, 18 May 2020 22:04:35 -0700 | From | Sean Christopherson <> | Subject | Re: [PATCH] x86/mm: Don't try to change poison pages to uncacheable in a guest |
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On Mon, May 18, 2020 at 06:55:00PM +0200, Borislav Petkov wrote: > On Mon, May 18, 2020 at 08:36:25AM -0700, Luck, Tony wrote: > > The VMM gets the page fault (because the unmapping of the guest > > physical address is at the VMM EPT level). The VMM can't map a new > > page into that guest physical address because it has no way to > > replace the contents of the old page. The VMM could pass the #PF > > to the guest, but that would just confuse the guest (its page tables > > all say that the page is still valid). In this particular case the > > page is part of the 1:1 kernel map. So the kernel will OOPS (I think). > > ... > > > PLease explain how a guest (that doesn't even know that it is a guest) > > is going to figure out that the EPT tables (that it has no way to access) > > have marked this page invalid in guest physical address space. > > So somewhere BUS_MCEERR_AR was mentioned. So I'm assuming the error > severity was "action required". What does happen in the kernel, on > baremetal, with an AR error in kernel space, i.e., kernel memory? > > If we can't fixup the exception, we die. > > So why should the guest behave any differently? > > Now, if you want for the guest to be more "robust" and handle that > thing, fine. But then you'd need an explicit way to tell the guest > kernel: "you've just had an MCE and I unmapped the page" so that the > guest kernel can figure out what do to. Even if it means, to panic. > > I.e., signal in an explicit way that EPT violation Jue is talking about > in the other mail.
Well, technically the CLFUSH thing is a KVM emulation bug, but it sounds like that's a moot point since the pmem-enabled guest will make real accesses to the poisoned page shortly thereafter. E.g. teaching KVM to eat the -EHWPOISON on CLFLUSH would only postpone the guest's death.
As for how the second #MC occurs, on the EPT violation, KVM does a gup() to translate the virtual address to a pfn (KVM maintains a simple GPA->HVA lookup). gup() returns -EHWPOISON for the poisoned page, which KVM redirects into a BUS_MCEERR_AR. The userspace VMM, e.g. Qemu, sees the BUS_MCEERR_AR and sends it back into the guest as a virtual #MC.
> You can inject a #PF or better yet the *first* MCE which is being > injected should say with a bit somehwere "I unmapped the address in > m->addr". So that the guest kernel can handle that properly and know > what *exactly* it is getting an MCE for. > > What I don't like is the "am I running as a guest" check. Because > someone else would come later and say, err, I'm not virtualizing this > portion of MCA either, lemme add another "am I guest" check. > > Sure, it is a lot easier but when stuff like that starts spreading > around in the MCE code, then we can just as well disable MCE when > virtualized altogether. It would be a lot easier for everybody.
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