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

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Date: Fri, 12 Apr 2024 13:17:02 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: isaku.yamahata@...el.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, 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,
	isaku.yamahata@...ux.intel.com
Subject: Re: [PATCH v19 079/130] KVM: TDX: vcpu_run: save/restore host
 state(host kernel gs)

On Sun, Apr 07, 2024 at 11:02:52AM +0800,
Binbin Wu <binbin.wu@...ux.intel.com> wrote:

> > 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?
>

No. Probably we should update the comment. Something like
  restored => restored or initialized to reset state.

Except conditionally saved/restored MSRs (e.g., perfrmon, debugreg), IA32_START,
IA32_LSTART, MSR_SYSCALL_MASK, IA32_TSC_AUX and TA32_KERNEL_GS_BASE are reset to
initial state.  uret handles the first four. The kernel_gs_base needs to be
restored on TDExit.

> >   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.

prepare_switch_to_guest() is used for kvm_x86_ops.prepare_switch_to_guest().
kvm_x86_ops consistently takes struct kvm_vcpu.
-- 
Isaku Yamahata <isaku.yamahata@...el.com>