Messages in this thread |  | | Date | Fri, 12 Feb 2021 12:44:55 -0800 | From | Sean Christopherson <> | Subject | Re: [RFC v1 05/26] x86/traps: Add #VE support for TDX guest |
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On Fri, Feb 12, 2021, Andy Lutomirski wrote: > > > On Feb 12, 2021, at 12:06 PM, Sean Christopherson <seanjc@google.com> wrote: > > > > On Fri, Feb 12, 2021, Andy Lutomirski wrote: > >>> On Fri, Feb 5, 2021 at 3:39 PM Kuppuswamy Sathyanarayanan > >>> <sathyanarayanan.kuppuswamy@linux.intel.com> wrote: > >>> > >>> From: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> > >>> > >>> The TDX module injects #VE exception to the guest TD in cases of > >>> disallowed instructions, disallowed MSR accesses and subset of CPUID > >>> leaves. Also, it's theoretically possible for CPU to inject #VE > >>> exception on EPT violation, but the TDX module makes sure this does > >>> not happen, as long as all memory used is properly accepted using > >>> TDCALLs. > >> > >> By my very cursory reading of the TDX arch specification 9.8.2, > >> "Secure" EPT violations don't send #VE. But the docs are quite > >> unclear, or at least the docs I found are. > > > > The version I have also states that SUPPRESS_VE is always set. So either there > > was a change in direction, or the public docs need to be updated. Lazy accept > > requires a #VE, either from hardware or from the module. The latter would > > require walking the Secure EPT tables on every EPT violation... > > > >> What happens if the guest attempts to access a secure GPA that is not > >> ACCEPTed? For example, suppose the VMM does THH.MEM.PAGE.REMOVE on a secure > >> address and the guest accesses it, via instruction fetch or data access. > >> What happens? > > > > Well, as currently written in the spec, it will generate an EPT violation and > > the host will have no choice but to kill the guest. > > Or page the page back in and try again?
The intended use isn't for swapping a page or migrating a page. Those flows have dedicated APIs, and do not _remove_ a page.
E.g. the KVM RFC patches already support zapping Secure EPT entries if NUMA balancing kicks in. But, in TDX terminology, that is a BLOCK/UNBLOCK operation.
Removal is for converting a private page to a shared page, and for paravirt memory ballooning.
> In regular virt guests, if the host pages out a guest page, it’s the host’s > job to put it back when needed. In paravirt, a well designed async of > protocol can sometimes let the guest to useful work when this happens. If a > guest (or bare metal) has its memory hot removed (via balloon or whatever) > and the kernel messes up and accesses removed memory, the guest (or bare > metal) is toast. > > I don’t see why TDX needs to be any different.
The REMOVE API isn't intended for swap. In fact, it can't be used for swap. If a page is removed, its contents are lost. Because the original contents are lost, the guest is required to re-accept the page so that the host can't silently get the guest to consume a zero page that the guest thinks has valid data.
For swap, the contents are preserved, and so explicit re-acceptance is not required. From the guest's perspective, it's really just a high-latency memory access.
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