On 12/16/21 06:36, Tian, Kevin wrote:
2) Do expansion at vCPU creation or KVM_ SET_CPUID2?
If the reallocation concept is still kept, then we feel doing expansion in
KVM_SET_CPUID2 makes slightly more sense. There is no functional
difference between two options since the guest is not running at this
point. And in general Qemu should set prctl according to the cpuid bits.
But since anyway we still need to check guest cpuid against guest perm in
KVM_SET_CPUID2, it reads clearer to expand the buffer only after this
check is passed.
Yes, that makes sense to me as well. In principle userspace could call
prctl only after KVM_CREATE_VCPU.
One option is to always disable WRMSR interception once
KVM_SET_CPUID2 succeeds, with the cost of one RDMSR per vm-exit.
But doing so affects legacy OS which even has no XFD logic at all.
The other option is to continue the current policy i.e. disable write
emulation only after the 1st interception of setting XFD to a non-zero
value. Then the RDMSR cost is added only for guest which supports XFD.
For this I suggest to implement the current policy, but place it at the
end of the series so it's easy to drop it.
Paolo
