Re: [PATCH v19 079/130] KVM: TDX: vcpu_run: save/restore host state(host kernel gs)

Binbin Wu
2024-04-07T03:02:58Z

On 2/26/2024 4:26 PM, isaku.yamahata@intel.com wrote:
> From: Isaku Yamahata <isaku.yamahata@intel.com>
>
> On entering/exiting TDX vcpu, Preserved or clobbered CPU state is different
> from VMX case.

Could you add more descriptions about the differences?

> Add TDX hooks to save/restore host/guest CPU state.

KVM doesn't save/restore guest CPU state for TDX.

> Save/restore kernel GS base MSR.
>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> arch/x86/kvm/vmx/main.c | 30 +++++++++++++++++++++++++--
> arch/x86/kvm/vmx/tdx.c | 42 ++++++++++++++++++++++++++++++++++++++
> arch/x86/kvm/vmx/tdx.h | 4 ++++
> arch/x86/kvm/vmx/x86_ops.h | 4 ++++
> 4 files changed, 78 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/main.c b/arch/x86/kvm/vmx/main.c
> index d72651ce99ac..8275a242ce07 100644
> --- a/arch/x86/kvm/vmx/main.c
> +++ b/arch/x86/kvm/vmx/main.c
> @@ -158,6 +158,32 @@ static void vt_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
> vmx_vcpu_reset(vcpu, init_event);
> }
>
> +static void vt_prepare_switch_to_guest(struct kvm_vcpu *vcpu)
> +{
> + /*
> + * All host state is saved/restored across SEAMCALL/SEAMRET,

It sounds confusing to me.
If all host states are saved/restored across SEAMCALL/SEAMRET, why this
patch saves/restores MSR_KERNEL_GS_BASE for host?

> and the
> + * guest state of a TD is obviously off limits. Deferring MSRs and DRs
> + * is pointless because the TDX module needs to load *something* so as
> + * not to expose guest state.
> + */
> + if (is_td_vcpu(vcpu)) {
> + tdx_prepare_switch_to_guest(vcpu);
> + return;
> + }
> +
> + vmx_prepare_switch_to_guest(vcpu);
> +}
> +
> +static void vt_vcpu_put(struct kvm_vcpu *vcpu)
> +{
> + if (is_td_vcpu(vcpu)) {
> + tdx_vcpu_put(vcpu);
> + return;
> + }
> +
> + vmx_vcpu_put(vcpu);
> +}
> +
> static int vt_vcpu_pre_run(struct kvm_vcpu *vcpu)
> {
> if (is_td_vcpu(vcpu))
> @@ -326,9 +352,9 @@ struct kvm_x86_ops vt_x86_ops __initdata = {
> .vcpu_free = vt_vcpu_free,
> .vcpu_reset = vt_vcpu_reset,
>
> - .prepare_switch_to_guest = vmx_prepare_switch_to_guest,
> + .prepare_switch_to_guest = vt_prepare_switch_to_guest,
> .vcpu_load = vmx_vcpu_load,
> - .vcpu_put = vmx_vcpu_put,
> + .vcpu_put = vt_vcpu_put,
>
> .update_exception_bitmap = vmx_update_exception_bitmap,
> .get_msr_feature = vmx_get_msr_feature,
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index fdf9196cb592..9616b1aab6ce 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -1,5 +1,6 @@
> // SPDX-License-Identifier: GPL-2.0
> #include <linux/cpu.h>
> +#include <linux/mmu_context.h>
>
> #include <asm/tdx.h>
>
> @@ -423,6 +424,7 @@ u8 tdx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio)
> int tdx_vcpu_create(struct kvm_vcpu *vcpu)
> {
> struct kvm_tdx *kvm_tdx = to_kvm_tdx(vcpu->kvm);
> + struct vcpu_tdx *tdx = to_tdx(vcpu);
>
> WARN_ON_ONCE(vcpu->arch.cpuid_entries);
> WARN_ON_ONCE(vcpu->arch.cpuid_nent);
> @@ -446,9 +448,47 @@ int tdx_vcpu_create(struct kvm_vcpu *vcpu)
> if ((kvm_tdx->xfam & XFEATURE_MASK_XTILE) == XFEATURE_MASK_XTILE)
> vcpu->arch.xfd_no_write_intercept = true;
>
> + tdx->host_state_need_save = true;
> + tdx->host_state_need_restore = false;
> +
> return 0;
> }
>
> +void tdx_prepare_switch_to_guest(struct kvm_vcpu *vcpu)

Just like vmx_prepare_switch_to_host(), the input can be "struct
vcpu_tdx *", since vcpu is not used inside the function.
And the callsites just use "to_tdx(vcpu)"

> +{
> + struct vcpu_tdx *tdx = to_tdx(vcpu);
Then, this can be dropped.

> +
> + if (!tdx->host_state_need_save)
> + return;
> +
> + if (likely(is_64bit_mm(current->mm)))
> + tdx->msr_host_kernel_gs_base = current->thread.gsbase;
> + else
> + tdx->msr_host_kernel_gs_base = read_msr(MSR_KERNEL_GS_BASE);
> +
> + tdx->host_state_need_save = false;
> +}
> +
> +static void tdx_prepare_switch_to_host(struct kvm_vcpu *vcpu)

ditto

> +{
> + struct vcpu_tdx *tdx = to_tdx(vcpu);
> +
> + tdx->host_state_need_save = true;
> + if (!tdx->host_state_need_restore)
> + return;
> +
> + ++vcpu->stat.host_state_reload;
> +
> + wrmsrl(MSR_KERNEL_GS_BASE, tdx->msr_host_kernel_gs_base);
> + tdx->host_state_need_restore = false;
> +}
> +
> +void tdx_vcpu_put(struct kvm_vcpu *vcpu)
> +{
> + vmx_vcpu_pi_put(vcpu);
> + tdx_prepare_switch_to_host(vcpu);
> +}
> +
> void tdx_vcpu_free(struct kvm_vcpu *vcpu)
> {
> struct vcpu_tdx *tdx = to_tdx(vcpu);
> @@ -569,6 +609,8 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu)
>
> tdx_vcpu_enter_exit(tdx);
>
> + tdx->host_state_need_restore = true;
> +
> vcpu->arch.regs_avail &= ~VMX_REGS_LAZY_LOAD_SET;
> trace_kvm_exit(vcpu, KVM_ISA_VMX);
>
> diff --git a/arch/x86/kvm/vmx/tdx.h b/arch/x86/kvm/vmx/tdx.h
> index 81d301fbe638..e96c416e73bf 100644
> --- a/arch/x86/kvm/vmx/tdx.h
> +++ b/arch/x86/kvm/vmx/tdx.h
> @@ -69,6 +69,10 @@ struct vcpu_tdx {
>
> bool initialized;
>
> + bool host_state_need_save;
> + bool host_state_need_restore;
> + u64 msr_host_kernel_gs_base;
> +
> /*
> * Dummy to make pmu_intel not corrupt memory.
> * TODO: Support PMU for TDX. Future work.
> diff --git a/arch/x86/kvm/vmx/x86_ops.h b/arch/x86/kvm/vmx/x86_ops.h
> index 3e29a6fe28ef..9fd997c79c33 100644
> --- a/arch/x86/kvm/vmx/x86_ops.h
> +++ b/arch/x86/kvm/vmx/x86_ops.h
> @@ -151,6 +151,8 @@ int tdx_vcpu_create(struct kvm_vcpu *vcpu);
> void tdx_vcpu_free(struct kvm_vcpu *vcpu);
> void tdx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event);
> fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu);
> +void tdx_prepare_switch_to_guest(struct kvm_vcpu *vcpu);
> +void tdx_vcpu_put(struct kvm_vcpu *vcpu);
> u8 tdx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio);
>
> int tdx_vcpu_ioctl(struct kvm_vcpu *vcpu, void __user *argp);
> @@ -186,6 +188,8 @@ static inline int tdx_vcpu_create(struct kvm_vcpu *vcpu) { return -EOPNOTSUPP; }
> static inline void tdx_vcpu_free(struct kvm_vcpu *vcpu) {}
> static inline void tdx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event) {}
> static inline fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu) { return EXIT_FASTPATH_NONE; }
> +static inline void tdx_prepare_switch_to_guest(struct kvm_vcpu *vcpu) {}
> +static inline void tdx_vcpu_put(struct kvm_vcpu *vcpu) {}
> static inline u8 tdx_get_mt_mask(struct kvm_vcpu *vcpu, gfn_t gfn, bool is_mmio) { return 0; }
>
> static inline int tdx_vcpu_ioctl(struct kvm_vcpu *vcpu, void __user *argp) { return -EOPNOTSUPP; }