Messages in this thread | | | Date | Tue, 08 Jun 2021 21:32:26 -0700 | From | "Andy Lutomirski" <> | Subject | Re: [RFC v2-fix-v4 1/1] x86/tdx: Skip WBINVD instruction for T DX guest |
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On Tue, Jun 8, 2021, at 9:25 PM, Andi Kleen wrote: > > > I like this description, but shouldn't the logic be: > > > > if (!CPUID has hypervisor bit set) > > wbinvd(); > > > > As far as I know, most hypervisors will turn WBINVD into a noop and, > > even if they don't, it seems to be that something must be really quite > > wrong for a guest to need to WBINVD for ACPI purposes. > > KVM only turns it into a noop if there is no VT-d, because with VT-d you > might need it to turn mappings into uncached and vice versa.
Wow, I found the kvm_arch_register_noncoherent_dma() stuff. That's horrifying. What's it for? Are there actually guests that use devices exposed by VFIO that expect WBINVD to work? That's a giant DoS hole.
> > But yes the change would make sense for reboot. BTW I suspect for the > reboot path it isn't really needed anywhere modern, so it might actually > be ok to completely disable it. But that's some risk, so doing it only > for hypervisor is reasonable.
I agree.
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