linux-kernel - Re: [PATCH v10 077/108] KVM: x86: Add a switch_db_regs flag to handle TDX's auto-switched behavior

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Message-ID: <20221216011223.GA527635@ls.amr.corp.intel.com>
Date:   Thu, 15 Dec 2022 17:12:23 -0800
From:   Isaku Yamahata <isaku.yamahata@...il.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>,
        David Matlack <dmatlack@...gle.com>,
        Xiaoyao Li <xiaoyao.li@...el.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Chao Gao <chao.gao@...el.com>
Subject: Re: [PATCH v10 077/108] KVM: x86: Add a switch_db_regs flag to
 handle TDX's auto-switched behavior

On Wed, Nov 16, 2022 at 10:41:34AM +0800,
Binbin Wu <binbin.wu@...ux.intel.com> wrote:

> 
> On 10/30/2022 2:23 PM, isaku.yamahata@...el.com wrote:
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> > 
> > Add a flag, KVM_DEBUGREG_AUTO_SWITCHED_GUEST, to skip saving/restoring DRs
> > irrespective of any other flags.  TDX-SEAM unconditionally saves and
> > restores guest DRs and reset to architectural INIT state on TD exit.
> > So, KVM needs to save host DRs before TD enter without restoring guest DRs
> > and restore host DRs after TD exit.
> > 
> > Opportunistically convert the KVM_DEBUGREG_* definitions to use BIT().
> > 
> > Reported-by: Xiaoyao Li <xiaoyao.li@...el.com>
> > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> > Co-developed-by: Chao Gao <chao.gao@...el.com>
> > Signed-off-by: Chao Gao <chao.gao@...el.com>
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> > ---
> >   arch/x86/include/asm/kvm_host.h |  9 +++++++--
> >   arch/x86/kvm/vmx/tdx.c          |  1 +
> >   arch/x86/kvm/x86.c              | 11 ++++++++---
> >   3 files changed, 16 insertions(+), 5 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index fdb00d96e954..082e94f78c66 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -583,8 +583,13 @@ struct kvm_pmu {
> >   struct kvm_pmu_ops;
> >   enum {
> > -	KVM_DEBUGREG_BP_ENABLED = 1,
> > -	KVM_DEBUGREG_WONT_EXIT = 2,
> > +	KVM_DEBUGREG_BP_ENABLED		= BIT(0),
> > +	KVM_DEBUGREG_WONT_EXIT		= BIT(1),
> > +	/*
> > +	 * Guest debug registers are saved/restored by hardware on exit from
> > +	 * or enter guest. KVM needn't switch them.
> > +	 */
> > +	KVM_DEBUGREG_AUTO_SWITCH	= BIT(2),
> >   };
> >   struct kvm_mtrr_range {
> > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > index fc4de83a2df8..57767ef3353b 100644
> > --- a/arch/x86/kvm/vmx/tdx.c
> > +++ b/arch/x86/kvm/vmx/tdx.c
> > @@ -429,6 +429,7 @@ int tdx_vcpu_create(struct kvm_vcpu *vcpu)
> >   	vcpu->arch.efer = EFER_SCE | EFER_LME | EFER_LMA | EFER_NX;
> > +	vcpu->arch.switch_db_regs = KVM_DEBUGREG_AUTO_SWITCH;
> >   	vcpu->arch.cr0_guest_owned_bits = -1ul;
> >   	vcpu->arch.cr4_guest_owned_bits = -1ul;
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 4d4b71c4cdb1..ad7b227b68dd 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -10779,7 +10779,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >   	if (vcpu->arch.guest_fpu.xfd_err)
> >   		wrmsrl(MSR_IA32_XFD_ERR, vcpu->arch.guest_fpu.xfd_err);
> > -	if (unlikely(vcpu->arch.switch_db_regs)) {
> > +	if (unlikely(vcpu->arch.switch_db_regs & ~KVM_DEBUGREG_AUTO_SWITCH)) {
> >   		set_debugreg(0, 7);
> >   		set_debugreg(vcpu->arch.eff_db[0], 0);
> >   		set_debugreg(vcpu->arch.eff_db[1], 1);
> > @@ -10822,6 +10822,7 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >   	 */
> >   	if (unlikely(vcpu->arch.switch_db_regs & KVM_DEBUGREG_WONT_EXIT)) {
> >   		WARN_ON(vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP);
> > +		WARN_ON(vcpu->arch.switch_db_regs & KVM_DEBUGREG_AUTO_SWITCH);
> >   		static_call(kvm_x86_sync_dirty_debug_regs)(vcpu);
> >   		kvm_update_dr0123(vcpu);
> >   		kvm_update_dr7(vcpu);
> > @@ -10834,8 +10835,12 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >   	 * care about the messed up debug address registers. But if
> >   	 * we have some of them active, restore the old state.
> >   	 */
> > -	if (hw_breakpoint_active())
> > -		hw_breakpoint_restore();
> > +	if (hw_breakpoint_active()) {
> > +		if (!(vcpu->arch.switch_db_regs & KVM_DEBUGREG_AUTO_SWITCH))
> > +			hw_breakpoint_restore();
> > +		else
> > +			set_debugreg(__this_cpu_read(cpu_dr7), 7);
> 
> Why only restore dr7 when TD exit?
> 
> According to the commit message, dr0~dr3 are also reset to architectural
> INIT value on TD exit.

Probably you're referring to the old version of the spec.
context switched: DR0-3, DR6, IA32_DS_AREA
reset on TD exit: RFLAGS, IA32_DEBUGCTL, DR7

I'll add a comment.
-- 
Isaku Yamahata <isaku.yamahata@...il.com>