Re: [PATCH v19 080/130] KVM: TDX: restore host xsave state when exit from the guest TD

[Chen Yu]

On 2024-02-26 at 00:26:22 -0800, isaku.yamahata@xxxxxxxxx wrote:
> From: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> 
> On exiting from the guest TD, xsave state is clobbered.  Restore xsave
> state on TD exit.
> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> ---
> v19:
> - Add EXPORT_SYMBOL_GPL(host_xcr0)
> 
> v15 -> v16:
> - Added CET flag mask
> 
> Signed-off-by: Isaku Yamahata <isaku.yamahata@xxxxxxxxx>
> ---
>  arch/x86/kvm/vmx/tdx.c | 19 +++++++++++++++++++
>  arch/x86/kvm/x86.c     |  1 +
>  2 files changed, 20 insertions(+)
> 
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 9616b1aab6ce..199226c6cf55 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -2,6 +2,7 @@
>  #include <linux/cpu.h>
>  #include <linux/mmu_context.h>
>  
> +#include <asm/fpu/xcr.h>
>  #include <asm/tdx.h>
>  
>  #include "capabilities.h"
> @@ -534,6 +535,23 @@ void tdx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>  	 */
>  }
>  
> +static void tdx_restore_host_xsave_state(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_tdx *kvm_tdx = to_kvm_tdx(vcpu->kvm);
> +
> +	if (static_cpu_has(X86_FEATURE_XSAVE) &&
> +	    host_xcr0 != (kvm_tdx->xfam & kvm_caps.supported_xcr0))
> +		xsetbv(XCR_XFEATURE_ENABLED_MASK, host_xcr0);
> +	if (static_cpu_has(X86_FEATURE_XSAVES) &&
> +	    /* PT can be exposed to TD guest regardless of KVM's XSS support */
> +	    host_xss != (kvm_tdx->xfam &
> +			 (kvm_caps.supported_xss | XFEATURE_MASK_PT | TDX_TD_XFAM_CET)))
> +		wrmsrl(MSR_IA32_XSS, host_xss);
> +	if (static_cpu_has(X86_FEATURE_PKU) &&
> +	    (kvm_tdx->xfam & XFEATURE_MASK_PKRU))
> +		write_pkru(vcpu->arch.host_pkru);
> +}

Maybe one minor question regarding the pkru restore. In the non-TDX version
kvm_load_host_xsave_state(), it first tries to read the current setting
vcpu->arch.pkru = rdpkru(); if this setting does not equal to host_pkru,
it trigger the write_pkru on host. Does it mean we can also leverage that mechanism
in TDX to avoid 1 pkru write(I guess pkru write is costly than a read pkru)?

thanks,
Chenyu