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Message-ID: <20240416164432.GZ3039520@ls.amr.corp.intel.com>
Date: Tue, 16 Apr 2024 09:44:32 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: "Huang, Kai" <kai.huang@...el.com>
Cc: "Yamahata, Isaku" <isaku.yamahata@...el.com>,
Sean Christopherson <seanjc@...gle.com>,
"Chatre, Reinette" <reinette.chatre@...el.com>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
Paolo Bonzini <pbonzini@...hat.com>,
"Aktas, Erdem" <erdemaktas@...gle.com>,
Sagi Shahar <sagis@...gle.com>, "Chen, Bo2" <chen.bo@...el.com>,
"Yuan, Hang" <hang.yuan@...el.com>,
"Zhang, Tina" <tina.zhang@...el.com>,
"isaku.yamahata@...ux.intel.com" <isaku.yamahata@...ux.intel.com>
Subject: Re: [PATCH v19 087/130] KVM: TDX: handle vcpu migration over logical
processor
On Tue, Apr 16, 2024 at 12:05:31PM +1200,
"Huang, Kai" <kai.huang@...el.com> wrote:
>
>
> On 16/04/2024 10:48 am, Yamahata, Isaku wrote:
> > On Mon, Apr 15, 2024 at 06:49:35AM -0700,
> > Sean Christopherson <seanjc@...gle.com> wrote:
> >
> > > 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?
> >
> > 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.
>
> I kinda lost here.
>
> I thought in the current v19 code, you have already implemented this
> optimization?
>
> Or is this optimization totally different from what we discussed in an
> earlier patch?
>
> https://lore.kernel.org/lkml/8feaba8f8ef249950b629f3a8300ddfb4fbcf11c.camel@intel.com/
That's only the first step. We can optimize it further with multiple vCPUs
context.
--
Isaku Yamahata <isaku.yamahata@...el.com>
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