On Tue, Feb 16, 2021 at 05:48:29PM +0100, Paolo Bonzini wrote: > We should minimize the number of #VEs that we get, as they are very slow. > Could almost everything that can invoke a #VE go through pvops and be turned > into a TDCALL? And if so the same should be true for SEV-ES #VC as well. The problem with that is that it requires the guest to know what the hypervisor will intercept or what instruction will cause a #VE. I considered this paravirtualization for #VC, but stayed away from it for that exact reason. You can't easily know which MMIO-access will cause a #VE/#VC exception. Probing also doesn't work, as the Hypervisor can change that at runtime. There is just no decent way to handle that without taking the #VE/#VC. Or take 'hlt' for example, there are hypervisor configurations which don't intercept it. How do you know that from within the guest? > > I guess those could all be replaced direct TDCALLs, > > but the question remains whether this is possible with MSR accesses, means > > that the list of MSRs which will cause #VEs is statically defined and > > doesn't change between hypervisors. All in all this sounds hard to > > maintain and easy to break by unrelated changes. > > I would expect that all MSRs except for a handful (SPEC_CTRL/PRED_CMD, the > FS/GS/kernelGS bases, anything else?) would be redirect to TDCALL. You never know which HV your guest runs under and what it intercepts. It can certainly be made part of the Spec to only allow direct access to a given set of MSRs in a TDX guest and require to intercept everything else. But that Spec probably requires constant updating and will certainly cause compatibility headaches in the future. Regards, Joerg
