linux-kernel - Re: [RFC PATCH 2/2] KVM: TDX: Kick off vCPUs when SEAMCALL is busy during TD page removal

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Message-ID: <Z2OHYWZeHeKMHfDy@google.com>
Date: Wed, 18 Dec 2024 18:39:29 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: pbonzini@...hat.com, kvm@...r.kernel.org, dave.hansen@...ux.intel.com, 
	rick.p.edgecombe@...el.com, kai.huang@...el.com, adrian.hunter@...el.com, 
	reinette.chatre@...el.com, xiaoyao.li@...el.com, tony.lindgren@...el.com, 
	binbin.wu@...ux.intel.com, dmatlack@...gle.com, isaku.yamahata@...el.com, 
	isaku.yamahata@...il.com, nik.borisov@...e.com, linux-kernel@...r.kernel.org, 
	x86@...nel.org
Subject: Re: [RFC PATCH 2/2] KVM: TDX: Kick off vCPUs when SEAMCALL is busy
 during TD page removal

On Thu, Dec 19, 2024, Yan Zhao wrote:
> On Wed, Dec 18, 2024 at 08:10:48AM -0800, Sean Christopherson wrote:
> > On Wed, Dec 18, 2024, Yan Zhao wrote:
> > > > Anyways, I don't see any reason to make this an arch specific request.
> > > After making it non-arch specific, probably we need an atomic counter for the
> > > start/stop requests in the common helpers. So I just made it TDX-specific to
> > > keep it simple in the RFC.
> > 
> > Oh, right.  I didn't consider the complications with multiple users.  Hrm.
> > 
> > Actually, this doesn't need to be a request.  KVM_REQ_OUTSIDE_GUEST_MODE will
> > forces vCPUs to exit, at which point tdx_vcpu_run() can return immediately with
> > EXIT_FASTPATH_EXIT_HANDLED, which is all that kvm_vcpu_exit_request() does.  E.g.
> > have the zap side set wait_for_sept_zap before blasting the request to all vCPU,
> Hmm, the wait_for_sept_zap also needs to be set and unset in all vCPUs except
> the current one.

Why can't it be a VM-wide flag?  The current vCPU isn't going to do VP.ENTER, is
it?  If it is, I've definitely missed something :-)

> >         /* TDX exit handle takes care of this error case. */
> >         if (unlikely(tdx->state != VCPU_TD_STATE_INITIALIZED)) {
> >                 tdx->vp_enter_ret = TDX_SW_ERROR;
> > @@ -921,6 +924,9 @@ fastpath_t tdx_vcpu_run(struct kvm_vcpu *vcpu, bool force_immediate_exit)
> >                 return EXIT_FASTPATH_NONE;
> >         }
> >  
> > +       if (unlikely(to_kvm_tdx(vcpu->kvm)->wait_for_sept_zap))
> > +               return EXIT_FASTPATH_EXIT_HANDLED;
> > +
> >         trace_kvm_entry(vcpu, force_immediate_exit);
> >  
> >         if (pi_test_on(&tdx->pi_desc)) {
> Thanks for this suggestion.
> But what's the advantage of this checking wait_for_sept_zap approach?
> Is it to avoid introducing an arch specific request?

Yes, and unless I've missed something, "releasing" vCPUs can be done by clearing
a single variable.