linux-kernel - Re: [PATCH v3 20/25] KVM: TDX: Add macro to retry SEAMCALLs when forcing vCPUs out of guest

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Message-ID: <aP_F6tmzomRtdbpU@google.com>
Date: Mon, 27 Oct 2025 12:20:10 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
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Subject: Re: [PATCH v3 20/25] KVM: TDX: Add macro to retry SEAMCALLs when
 forcing vCPUs out of guest

On Fri, Oct 24, 2025, Kai Huang wrote:
> On Thu, 2025-10-16 at 17:32 -0700, Sean Christopherson wrote:
> > Add a macro to handle kicking vCPUs out of the guest and retrying
> > SEAMCALLs on -EBUSY instead of providing small helpers to be used by each
> > SEAMCALL.  Wrapping the SEAMCALLs in a macro makes it a little harder to
> > tease out which SEAMCALL is being made, but significantly reduces the
> > amount of copy+paste code and makes it all but impossible to leave an
> > elevated wait_for_sept_zap.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/kvm/vmx/tdx.c | 72 ++++++++++++++----------------------------
> >  1 file changed, 23 insertions(+), 49 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> > index f6782b0ffa98..2e2dab89c98f 100644
> > --- a/arch/x86/kvm/vmx/tdx.c
> > +++ b/arch/x86/kvm/vmx/tdx.c
> > @@ -294,25 +294,24 @@ static inline void tdx_disassociate_vp(struct kvm_vcpu *vcpu)
> >  	vcpu->cpu = -1;
> >  }
> >  
> > -static void tdx_no_vcpus_enter_start(struct kvm *kvm)
> > -{
> > -	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> > -
> > -	lockdep_assert_held_write(&kvm->mmu_lock);
> > -
> > -	WRITE_ONCE(kvm_tdx->wait_for_sept_zap, true);
> > -
> > -	kvm_make_all_cpus_request(kvm, KVM_REQ_OUTSIDE_GUEST_MODE);
> > -}
> > -
> > -static void tdx_no_vcpus_enter_stop(struct kvm *kvm)
> > -{
> > -	struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
> > -
> > -	lockdep_assert_held_write(&kvm->mmu_lock);
> > -
> > -	WRITE_ONCE(kvm_tdx->wait_for_sept_zap, false);
> > -}
> > +#define tdh_do_no_vcpus(tdh_func, kvm, args...)					\
> > +({										\
> > +	struct kvm_tdx *__kvm_tdx = to_kvm_tdx(kvm);				\
> > +	u64 __err;								\
> > +										\
> > +	lockdep_assert_held_write(&kvm->mmu_lock);				\
> > +										\
> > +	__err = tdh_func(args);							\
> > +	if (unlikely(tdx_operand_busy(__err))) {				\
> > +		WRITE_ONCE(__kvm_tdx->wait_for_sept_zap, true);			\
> > +		kvm_make_all_cpus_request(kvm, KVM_REQ_OUTSIDE_GUEST_MODE);	\
> > +										\
> > +		__err = tdh_func(args);						\
> > +										\
> > +		WRITE_ONCE(__kvm_tdx->wait_for_sept_zap, false);		\
> > +	}									\
> > +	__err;									\
> > +})
> 
> The comment which says "the second retry should succeed" is lost, could we
> add it to tdh_do_no_vcpus()?

+1, definitely needs a comment.

/*
 * Execute a SEAMCALL related to removing/blocking S-EPT entries, with a single
 * retry (if necessary) after forcing vCPUs to exit and wait for the operation
 * to complete.  All flows that remove/block S-EPT entries run with mmu_lock
 * held for write, i.e. are mutually exlusive with each other, but they aren't
 * mutually exclusive with vCPUs running (because that would be overkill), and
 * so can fail with "operand busy" if a vCPU acquires a required lock in the
 * TDX-Module.
 *
 * Note, the retry is guaranteed to succeed, absent KVM and/or TDX-Module bugs.
 */
 
> Also, perhaps we can just TDX_BUG_ON() inside tdh_do_no_vcpus() when the
> second call of tdh_func() fails?

Heh, this also caught my eye when typing up the comment.  Unfortunately, I don't
think it's worth doing the TDX_BUG_ON() inside the macro as that would require
plumbing in the UPPERCASE name, and doesn't work well with the variadic arguments,
e.g. TRACK wants TDX_BUG_ON(), but REMOVE and BLOCK want TDX_BUG_ON_2().

Given that REMOVE and BLOCK need to check the return value, getting the TDX_BUG_ON()
call into the macro wouldn't buy that much.