On Thu, Jul 20, 2023 at 09:25:14AM +0800, Xiaoyao Li wrote: >On 7/20/2023 2:08 AM, Jim Mattson wrote: >> Normally, we would restrict guest MSR writes based on guest CPU >> features. However, with IA32_SPEC_CTRL and IA32_PRED_CMD, this is not >> the case. This issue isn't specific to the two MSRs. Any MSRs that are not intercepted and with some reserved bits for future extenstions may run into this issue. Right? IMO, it is a conflict of interests between disabling MSR write intercept for less VM-exits and host's control over the value written to the MSR by guest. We may need something like CR0/CR4 masks and read shadows for all MSRs to address this fundamental issue. >> >> For the first non-zero write to IA32_SPEC_CTRL, we check to see that >> the host supports the value written. We don't care whether or not the >> guest supports the value written (as long as it supports the MSR). >> After the first non-zero write, we stop intercepting writes to >> IA32_SPEC_CTRL, so the guest can write any value supported by the >> hardware. This could be problematic in heterogeneous migration pools. >> For instance, a VM that starts on a Cascade Lake host may set >> IA32_SPEC_CTRL.PSFD[bit 7], even if the guest >> CPUID.(EAX=07H,ECX=02H):EDX.PSFD[bit 0] is clear. Then, if that VM is >> migrated to a Skylake host, KVM_SET_MSRS will refuse to set >> IA32_SPEC_CTRL to its current value, because Skylake doesn't support >> PSFD. It is a guest fault. Can we modify guest kernel in this case? >> >> We disable write intercepts IA32_PRED_CMD as long as the guest >> supports the MSR. That's fine for now, since only one bit of PRED_CMD >> has been defined. Hence, guest support and host support are >> equivalent...today. But, are we really comfortable with letting the >> guest set any IA32_PRED_CMD bit that may be defined in the future? >> >> The same question applies to IA32_SPEC_CTRL. Are we comfortable with >> letting the guest write to any bit that may be defined in the future? > >My point is we need to fix it, though Chao has different point that sometimes >performance may be more important[*] > >[*] https://lore.kernel.org/all/ZGdE3jNS11wV+V2w@chao-email/ Maybe KVM can provide options to QEMU. e.g., we can define a KVM quirk. Disabling the quirk means always intercept IA32_SPEC_CTRL MSR writes. > >> At least the AMD approach with V_SPEC_CTRL prevents the guest from >> clearing any bits set by the host, but on Intel, it's a total >> free-for-all. What happens when a new bit is defined that absolutely >> must be set to 1 all of the time? I suppose there is no such bit now. For SPR and future CPUs, "virtualize IA32_SPEC_CTRL" VMX feature can lock some bits to 0 or 1 regardless of the value written by guests.
