On Sat, Oct 12, 2024 at 12:55:54AM -0700, 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. OK, previous results were incorrect. In fact, this patches (which apply cleanly to current kvm-coco-queue) reduce cyclictest latency from: Max Latencies: 00167 00160 Max Latencies: 00132 00151 Max Latencies: 00138 00142 Max Latencies: 02512 02582 Max Latencies: 00139 00140 Max Latencies: 00128 00131 Max Latencies: 00131 00132 Max Latencies: 00131 00134 Max Latencies: 00136 00147 Max Latencies: 00153 00135 Max Latencies: 00138 00138 to: Max Latencies: 00134 00131 Max Latencies: 00130 00129 Max Latencies: 00126 00141 Max Latencies: 00137 00138 Max Latencies: 00123 00115 Max Latencies: 00119 00127 Max Latencies: 00131 00104 Max Latencies: 00137 00127 Max Latencies: 00135 00126 Max Latencies: 00128 00142 Max Latencies: 00135 00138 > > > 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. > > > 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. > > > Patches: > 1: Preparation for the next patch > 2: Skip the logic to adjust the TSC offset/multiplier in the common x86 KVM logic > > [1] https://git.kernel.org/pub/scm/utils/rt-tests/rt-tests.git > > Changes for TDX KVM > > 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 { > > --- > Isaku Yamahata (2): > KVM: x86: Push down setting vcpu.arch.user_set_tsc > KVM: x86: Don't allow tsc_offset, tsc_scaling_ratio to change > > arch/x86/include/asm/kvm_host.h | 1 + > arch/x86/kvm/x86.c | 21 ++++++++++++++------- > 2 files changed, 15 insertions(+), 7 deletions(-) > > > base-commit: 909f9d422f59f863d7b6e4e2c6e57abb97a27d4d > -- > 2.45.2 > >
