Re: [PATCH V2 09/12] KVM: TDX: restore user ret MSRs

Xiaoyao Li
2025-02-25T07:02:06Z

On 1/29/2025 5:58 PM, Adrian Hunter wrote:
> From: Isaku Yamahata <isaku.yamahata@intel.com>
>
> Several user ret MSRs are clobbered on TD exit. Restore those values on
> TD exit and before returning to ring 3.
>
> Co-developed-by: Tony Lindgren <tony.lindgren@linux.intel.com>
> Signed-off-by: Tony Lindgren <tony.lindgren@linux.intel.com>
> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
> Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
> TD vcpu enter/exit v2:
> - No changes
>
> TD vcpu enter/exit v1:
> - Rename tdx_user_return_update_cache() ->
> tdx_user_return_msr_update_cache() (extrapolated from Binbin)
> - Adjust to rename in previous patches (Binbin)
> - Simplify comment (Tony)
> - Move code change in tdx_hardware_setup() to __tdx_bringup().
> ---
> arch/x86/kvm/vmx/tdx.c | 44 +++++++++++++++++++++++++++++++++++++++++-
> 1 file changed, 43 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index e4355553569a..a0f5cdfd290b 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -729,6 +729,28 @@ int tdx_vcpu_pre_run(struct kvm_vcpu *vcpu)
> return 1;
> }
>
> +struct tdx_uret_msr {
> + u32 msr;
> + unsigned int slot;
> + u64 defval;
> +};
> +
> +static struct tdx_uret_msr tdx_uret_msrs[] = {
> + {.msr = MSR_SYSCALL_MASK, .defval = 0x20200 },
> + {.msr = MSR_STAR,},
> + {.msr = MSR_LSTAR,},
> + {.msr = MSR_TSC_AUX,},
> +};
> +
> +static void tdx_user_return_msr_update_cache(void)
> +{
> + int i;

I think it can be optimized to skip update cache if it the caches are
updated already. No need to update the cache after every TD exit.

> + for (i = 0; i < ARRAY_SIZE(tdx_uret_msrs); i++)
> + kvm_user_return_msr_update_cache(tdx_uret_msrs[i].slot,
> + tdx_uret_msrs[i].defval);
> +}
> +
> static bool tdx_guest_state_is_invalid(struct kvm_vcpu *vcpu)
> {
> struct kvm_tdx *kvm_tdx = to_kvm_tdx(vcpu->kvm);
> @@ -784,6 +806,8 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu, bool force_immediate_exit)
>
> tdx_vcpu_enter_exit(vcpu);
>
> + tdx_user_return_msr_update_cache();
> +
> kvm_load_host_xsave_state(vcpu);
>
> vcpu->arch.regs_avail &= ~TDX_REGS_UNSUPPORTED_SET;
> @@ -2245,7 +2269,25 @@ static bool __init kvm_can_support_tdx(void)
> static int __init __tdx_bringup(void)
> {
> const struct tdx_sys_info_td_conf *td_conf;
> - int r;
> + int r, i;
> +
> + for (i = 0; i < ARRAY_SIZE(tdx_uret_msrs); i++) {
> + /*
> + * Check if MSRs (tdx_uret_msrs) can be saved/restored
> + * before returning to user space.
> + *
> + * this_cpu_ptr(user_return_msrs)->registered isn't checked
> + * because the registration is done at vcpu runtime by
> + * tdx_user_return_msr_update_cache().
> + */
> + tdx_uret_msrs[i].slot = kvm_find_user_return_msr(tdx_uret_msrs[i].msr);
> + if (tdx_uret_msrs[i].slot == -1) {
> + /* If any MSR isn't supported, it is a KVM bug */
> + pr_err("MSR %x isn't included by kvm_find_user_return_msr\n",
> + tdx_uret_msrs[i].msr);
> + return -EIO;
> + }
> + }
>
> /*
> * Enabling TDX requires enabling hardware virtualization first,