linux-kernel - Re: [PATCH v8 2/5] x86/tdx: Add TDX Guest event notify interrupt support

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Message-ID: <61af8186d3252440baf28706c46ef1a8edead3ba.camel@intel.com>
Date:   Thu, 23 Jun 2022 21:46:41 +1200
From:   Kai Huang <kai.huang@...el.com>
To:     Sathyanarayanan Kuppuswamy 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org
Cc:     "H . Peter Anvin" <hpa@...or.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Wander Lairson Costa <wander@...hat.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        marcelo.cerri@...onical.com, tim.gardner@...onical.com,
        khalid.elmously@...onical.com, philip.cox@...onical.com,
        linux-kernel@...r.kernel.org,
        "Nakajima, Jun" <jun.nakajima@...el.com>
Subject: Re: [PATCH v8 2/5] x86/tdx: Add TDX Guest event notify interrupt
 support

> 
> > 
> > That being said, if a TD has multiple devices, it cannot know whether the VMM
> > will inject the removal event via the vector set by SetupEventNotifyInterrupt. 
> > And for the same device in the same TD, different VMMs may use different way to
> > notify its removal.
> 
> As per current design, If it is used for device removal, I think all registered
> device drivers will get the notification and the individual device driver has
> to check whether it is applicable for them.

The problem is there's no _specification_ around this.  As I said above, I don't
see why TDX architecture cannot support Qemu enumerated ACPI-based hotplug,
which uses SCI.  Maybe I am missing something here, but IMHO we should have some
_specification_ around SetupEventNotifyInterrupt in GHCI.

> 
> If the SetupEventNotifyInterrupt TDVMCALL specification is extended to specify
> the exact device or use case detail, then it can optimize the implementation.
> 
> > 
> > It seems GetQuote is the only user of SetupEventNotifyInterrupt.  Maybe we
> > should just declare it is for GetQuote.
> 
> Ok.

If you believe this is reasonable, perhaps you can drive the spec change.

> 
> > 
> > Isaku, what do you think?  Does this make sense?
> > 
> > > 
> > > In TDX guest, SetupEventNotifyInterrupt hypercall can be used by the
> > > guest to specify which interrupt vector to use as an event-notify
> > > vector to the VMM. Details about the SetupEventNotifyInterrupt
> > > hypercall can be found in TDX Guest-Host Communication Interface
> > > (GHCI) Specification, sec 3.5 "VP.VMCALL<SetupEventNotifyInterrupt>".
> > > Add a tdx_hcall_set_notify_intr() helper function to implement the
> > > SetupEventNotifyInterrupt hypercall.
> > 
> > As you also used "can" above, the GHCI only says the VMM _CAN_ inject the vector
> > set by SetupEventNotifyInterrupt, but not must (3.3 TDG.VP.VMCALL<GetQuote>). 
> > This means theoretically TD should implement pooling mode in case VMM doesn't
> > support injecting event via vector done by SetupEventNotifyInterrupt?
> 
> Yes. But GetQuote specification does not talk about the pooling mode
> use case as well. So I think it is just a wording confusion.

It doesn't need to mention I think.  "can" means VMM can choose to inject or
not, but not must, which basically implies GetQuote should support pooling.

> 
> > 
> > Perhaps we should update the GHCI spec to use must..
> 
> Ok.

If you don't want to support pooling, I guess you'd better to improve the GHCI.

[...]

> 
> 
> > resource especially on server systems with a lot of CPUs.
> 
> FWIW, this reservation is protected with CONFIG_INTEL_TDX_GUEST. So it will be
> reserved only for TDX use case.

This reason doesn't stand.  I think distributions basically tends to enable all
Kconfig options so one binary works on all machines, so it can be (maybe likely)
turned on even on bare-metal machines.

-- 
Thanks,
-Kai