| Message ID | cover.1728719037.git.isaku.yamahata@intel.com (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
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
diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c index 8785309ccb46..969da729d89f 100644 --- a/arch/x86/kvm/vmx/tdx.c +++ b/arch/x86/kvm/vmx/tdx.c @@ -694,8 +712,6 @@ int tdx_vcpu_create(struct kvm_vcpu *vcpu) vcpu->arch.cr0_guest_owned_bits = -1ul; vcpu->arch.cr4_guest_owned_bits = -1ul; - vcpu->arch.tsc_offset = kvm_tdx->tsc_offset; - vcpu->arch.l1_tsc_offset = vcpu->arch.tsc_offset; /* * TODO: support off-TD debug. If TD DEBUG is enabled, guest state * can be accessed. guest_state_protected = false. and kvm ioctl to @@ -706,6 +722,13 @@ int tdx_vcpu_create(struct kvm_vcpu *vcpu) */ vcpu->arch.guest_state_protected = true; + /* VMM can't change TSC offset/multiplier as TDX module manages them. */ + vcpu->arch.guest_tsc_protected = true; + vcpu->arch.tsc_offset = kvm_tdx->tsc_offset; + vcpu->arch.l1_tsc_offset = vcpu->arch.tsc_offset; + vcpu->arch.tsc_scaling_ratio = kvm_tdx->tsc_multiplier; + vcpu->arch.l1_tsc_scaling_ratio = kvm_tdx->tsc_multiplier; + if ((kvm_tdx->xfam & XFEATURE_MASK_XTILE) == XFEATURE_MASK_XTILE) vcpu->arch.xfd_no_write_intercept = true; @@ -2674,6 +2697,7 @@ static int tdx_td_init(struct kvm *kvm, struct kvm_tdx_cmd *cmd) goto out; kvm_tdx->tsc_offset = td_tdcs_exec_read64(kvm_tdx, TD_TDCS_EXEC_TSC_OFFSET); + kvm_tdx->tsc_multiplier = td_tdcs_exec_read64(kvm_tdx, TD_TDCS_EXEC_TSC_MULTIPLIER); kvm_tdx->attributes = td_params->attributes; kvm_tdx->xfam = td_params->xfam; diff --git a/arch/x86/kvm/vmx/tdx.h b/arch/x86/kvm/vmx/tdx.h index 614b1c3b8483..c0e4fa61cab1 100644 --- a/arch/x86/kvm/vmx/tdx.h +++ b/arch/x86/kvm/vmx/tdx.h @@ -42,6 +42,7 @@ struct kvm_tdx { bool tsx_supported; u64 tsc_offset; + u64 tsc_multiplier; enum kvm_tdx_state state; diff --git a/arch/x86/kvm/vmx/tdx_arch.h b/arch/x86/kvm/vmx/tdx_arch.h index 861c0f649b69..be4cf65c90a8 100644 --- a/arch/x86/kvm/vmx/tdx_arch.h +++ b/arch/x86/kvm/vmx/tdx_arch.h @@ -69,6 +69,7 @@ enum tdx_tdcs_execution_control { TD_TDCS_EXEC_TSC_OFFSET = 10, + TD_TDCS_EXEC_TSC_MULTIPLIER = 11, }; enum tdx_vcpu_guest_other_state {