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

[Xiaoyao Li <xiaoyao.li@intel.com>]

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,