Hi Isaku, On 10/12/2024 1:25 PM, Isaku Yamahata wrote: > This patch series is for the kvm-coco-queue branch. The change for TDX KVM is > included at the last. The test is done by create TDX vCPU and run, get TSC > offset via vCPU device attributes and compare it with the TDX TSC OFFSET > metadata. Because the test requires the TDX KVM and TDX KVM kselftests, don't > include it in this patch series. > > > Background > ---------- > X86 confidential computing technology defines protected guest TSC so that the > VMM can't change the TSC offset/multiplier once vCPU is initialized and the > guest can trust TSC. The SEV-SNP defines Secure TSC as optional. TDX mandates > it. The TDX module determines the TSC offset/multiplier. The VMM has to > retrieve them. > > On the other hand, the x86 KVM common logic tries to guess or adjust the 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()). > > > Problem > ------- > The current x86 KVM implementation conflicts with protected TSC because the > VMM can't change the TSC offset/multiplier. Disable or ignore the KVM > logic to change/adjust the TSC offset/multiplier somehow. > > Because KVM emulates the TSC timer or the TSC deadline timer with the TSC > offset/multiplier, the TSC timer interrupts are injected to the guest at the > wrong time if the KVM TSC offset is different from what the TDX module > determined. > > Originally the 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. Can you provide what is the exact command line to reproduce this problem ? Any links to this reported issue ? > > > Solution > -------- > The solution is to keep the KVM TSC offset/multiplier the same as the value of > the TDX module somehow. Possible solutions are as follows. > - Skip the logic > Ignore (or don't call related functions) the request to change the TSC > offset/multiplier. > Pros > - Logically clean. This is similar to the guest_protected case. > Cons > - Needs to identify the call sites. > > - Revert the change at the hooks after TSC adjustment > x86 KVM defines the vendor hooks when the TSC offset/multiplier are > changed. The callback can revert the change. > Pros > - We don't need to care about the logic to change the TSC offset/multiplier. > Cons: > - Hacky to revert the KVM x86 common code logic. > > Choose the first one. With this patch series, SEV-SNP secure TSC can be > supported. I am not sure how will this help SNP Secure TSC, as the GUEST_TSC_OFFSET and GUEST_TSC_SCALE are only available to the guest. Regards, Nikunj
