linux-kernel - Re: [PATCH v19 086/130] KVM: TDX: restore debug store when TD exit

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Date: Tue, 16 Apr 2024 11:24:16 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: <isaku.yamahata@...el.com>, <kvm@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
CC: <isaku.yamahata@...il.com>, Paolo Bonzini <pbonzini@...hat.com>,
	<erdemaktas@...gle.com>, Sean Christopherson <seanjc@...gle.com>, Sagi Shahar
	<sagis@...gle.com>, Kai Huang <kai.huang@...el.com>, <chen.bo@...el.com>,
	<hang.yuan@...el.com>, <tina.zhang@...el.com>
Subject: Re: [PATCH v19 086/130] KVM: TDX: restore debug store when TD exit

Hi Isaku,

On 2/26/2024 12:26 AM, isaku.yamahata@...el.com wrote:
> From: Isaku Yamahata <isaku.yamahata@...el.com>
> 
> Because debug store is clobbered, restore it on TD exit.
> 

I am trying to understand why this is needed and finding a few
places that seem to indicate that this is not needed.

For example, in the TX base spec, section 15.2 "On-TD Performance
Monitoring", subsection "15.2.1 Overview":
 * IA32_DS_AREA MSR is context-switched across TD entry and exit transitions.

To confirm I peeked at the TDX module code and found (if I understand this
correctly) that IA32_DS_AREA is saved on TD entry (see [1]) and restored on 
TD exit (see [2]).


> Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> Reviewed-by: Paolo Bonzini <pbonzini@...hat.com>
> ---
>  arch/x86/events/intel/ds.c | 1 +
>  arch/x86/kvm/vmx/tdx.c     | 1 +
>  2 files changed, 2 insertions(+)
> 
> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
> index d49d661ec0a7..25670d8a485b 100644
> --- a/arch/x86/events/intel/ds.c
> +++ b/arch/x86/events/intel/ds.c
> @@ -2428,3 +2428,4 @@ void perf_restore_debug_store(void)
>  
>  	wrmsrl(MSR_IA32_DS_AREA, (unsigned long)ds);
>  }
> +EXPORT_SYMBOL_GPL(perf_restore_debug_store);
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index b8b168f74dfe..ad4d3d4eaf6c 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -665,6 +665,7 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu)
>  	tdx_vcpu_enter_exit(tdx);
>  
>  	tdx_user_return_update_cache(vcpu);
> +	perf_restore_debug_store();
>  	tdx_restore_host_xsave_state(vcpu);
>  	tdx->host_state_need_restore = true;
>  

Reinette

[1] https://github.com/intel/tdx-module/blob/tdx_1.5/src/td_transitions/tdh_vp_enter.c#L719
[2] https://github.com/intel/tdx-module/blob/tdx_1.5/src/td_transitions/td_exit.c#L130