KVM: x86: Add infrastructure for secure TSC

KVM: x86: Add infrastructure for secure TSC

Add guest_tsc_protected member to struct kvm_arch_vcpu and prohibit
changing TSC offset/multiplier when guest_tsc_protected is true.

X86 confidential computing technology defines protected guest TSC so that
the VMM can't change the TSC offset/multiplier once vCPU is initialized.
SEV-SNP defines Secure TSC as optional, whereas TDX mandates it.

KVM has common logic on x86 that tries to guess or adjust TSC
offset/multiplier for better guest TSC and TSC interrupt latency
at KVM vCPU creation (kvm_arch_vcpu_postcreate()), vCPU migration
over pCPU (kvm_arch_vcpu_load()), vCPU TSC device attributes
(kvm_arch_tsc_set_attr()) and guest/host writing to TSC or TSC adjust MSR
(kvm_set_msr_common()).

The current x86 KVM implementation conflicts with protected TSC because the
VMM can't change the TSC offset/multiplier.
Because KVM emulates the TSC timer or the TSC deadline timer with the TSC
offset/multiplier, the TSC timer interrupts is injected to the guest at the
wrong time if the KVM TSC offset is different from what the TDX module
determined.

Originally this issue was found by cyclic test of rt-test [1] as the
latency in TDX case is worse than VMX value + TDX SEAMCALL overhead.  It
turned out that the KVM TSC offset is different from what the TDX module
determines.

Disable or ignore the KVM logic to change/adjust the TSC offset/multiplier
somehow, thus keeping the KVM TSC offset/multiplier the same as the
value of the TDX module.  Writes to MSR_IA32_TSC are also blocked as
they amount to a change in the TSC offset.

[1] https://git.kernel.org/pub/scm/utils/rt-tests/rt-tests.git

Reported-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
Message-ID: <3a7444aec08042fe205666864b6858910e86aa98.1728719037.git.isaku.yamahata@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>