linux-kernel - Re: [PATCH v19 087/130] KVM: TDX: handle vcpu migration over logical processor

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Date: Mon, 15 Apr 2024 06:49:35 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Isaku Yamahata <isaku.yamahata@...el.com>
Cc: Reinette Chatre <reinette.chatre@...el.com>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, isaku.yamahata@...il.com, 
	Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...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 087/130] KVM: TDX: handle vcpu migration over logical processor

On Fri, Apr 12, 2024, Isaku Yamahata wrote:
> On Fri, Apr 12, 2024 at 03:46:05PM -0700,
> Sean Christopherson <seanjc@...gle.com> wrote:
> 
> > On Fri, Apr 12, 2024, Isaku Yamahata wrote:
> > > On Fri, Apr 12, 2024 at 09:15:29AM -0700, Reinette Chatre <reinette.chatre@...el.com> wrote:
> > > > > +void tdx_mmu_release_hkid(struct kvm *kvm)
> > > > > +{
> > > > > +	while (__tdx_mmu_release_hkid(kvm) == -EBUSY)
> > > > > +		;
> > > > >  }
> > > > 
> > > > As I understand, __tdx_mmu_release_hkid() returns -EBUSY
> > > > after TDH.VP.FLUSH has been sent for every vCPU followed by
> > > > TDH.MNG.VPFLUSHDONE, which returns TDX_FLUSHVP_NOT_DONE.
> > > > 
> > > > Considering earlier comment that a retry of TDH.VP.FLUSH is not
> > > > needed, why is this while() loop here that sends the
> > > > TDH.VP.FLUSH again to all vCPUs instead of just a loop within
> > > > __tdx_mmu_release_hkid() to _just_ resend TDH.MNG.VPFLUSHDONE?
> > > > 
> > > > Could it be possible for a vCPU to appear during this time, thus
> > > > be missed in one TDH.VP.FLUSH cycle, to require a new cycle of
> > > > TDH.VP.FLUSH?
> > > 
> > > Yes. There is a race between closing KVM vCPU fd and MMU notifier release hook.
> > > When KVM vCPU fd is closed, vCPU context can be loaded again.
> > 
> > But why is _loading_ a vCPU context problematic?
> 
> It's nothing problematic.  It becomes a bit harder to understand why
> tdx_mmu_release_hkid() issues IPI on each loop.  I think it's reasonable
> to make the normal path easy and to complicate/penalize the destruction path.
> Probably I should've added comment on the function.

By "problematic", I meant, why can that result in a "missed in one TDH.VP.FLUSH
cycle"?  AFAICT, loading a vCPU shouldn't cause that vCPU to be associated from
the TDX module's perspective, and thus shouldn't trigger TDX_FLUSHVP_NOT_DONE.

I.e. looping should be unnecessary, no?