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

[Isaku Yamahata]

On Mon, Apr 15, 2024 at 06:49:35AM -0700,
Sean Christopherson <seanjc@xxxxxxxxxx> wrote:

> On Fri, Apr 12, 2024, Isaku Yamahata wrote:
> > On Fri, Apr 12, 2024 at 03:46:05PM -0700,
> > Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > 
> > > On Fri, Apr 12, 2024, Isaku Yamahata wrote:
> > > > On Fri, Apr 12, 2024 at 09:15:29AM -0700, Reinette Chatre <reinette.chatre@xxxxxxxxx> 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?

The loop is unnecessary with the current code.

The possible future optimization is to reduce destruction time of Secure-EPT
somehow.  One possible option is to release HKID while vCPUs are still alive and
destruct Secure-EPT with multiple vCPU context.  Because that's future
optimization, we can ignore it at this phase.
-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>