Re: TDG quote analysis

[Sean Christopherson <seanjc@google.com>]

On Mon, Sep 21, 2026, Dave Hansen wrote:
> On 9/21/26 16:19, Sean Christopherson wrote:
> > On Mon, Sep 21, 2026, Dave Hansen wrote:
> >> On 9/18/26 14:32, Sean Christopherson wrote:
> >>> By creating these system-wide thread pools, TDX has effectively created a bizarre
> >>> M:N scheduling problem, *and* introduced a completely avoidable noisy-neighbor
> >>> problem.
> >> To be honest, I just want to chop all of the complexity out of this.
> >>
> >> I want to assume that the TDX module can do one quote at a time. If that
> >> quote takes too long, software kills it and lets the next guy do a quote.
> >>
> >> I don't even want to know that the TDX module *can* have 1 or 10 or 100
> >> quoting threads. I just want one which I can make sane rules around
> >> because 1 is *FINE*.
> > I'm not remotely convinced one per system is fine.  I'm all for one per VM, but
> > I'm pretty sure the SEAMCALL alone will have a measurable performance impact,
> > especially when factoring in that it will require a VMCS shootdown to do VMCLEAR
> > on the previous pCPU.  Mix in a few (tens of?) thousand more cycles, and any use
> > case that's trying to boot a decent number of TDX guests in parallel will be sad.
> 
> Sad, but way less sad than the folks who decided to offload their
> quoting to an Arduino attached over a fancy serial port. ;)
> 
> I do think we can _start_ with one. One needs no ABI and no policy bike
> shedding. If my crystal ball is right, we can stop there. If yours is
> right, then we can go build the stuff we need to make it scale.

My preference *was* to not have to find out, because those types of scaling issues
tend to lead to a mad scramble and angry customers.  But...

> Oh, and folks can _theoretically_ do one quoting instance per VM. But I
> think the overhead per quoting instance is in 100MB ballpark. 

MB with an M!?!?!  What!?!?   How?  I was assuming my 32KiB threshold was fairly
conservative; what is the TDX Module doing that it needs 100MiB?  That's insane.
That's literally 3x the footprint of some full-blown VMMs.

If it really is 100MiB per instance, that definitely changes things.  I'm much
more willing to roll the dice and hope quoting is never a bottleneck in practice
if it means saving 100MiB per VM.  Yeesh.

> So a bit rotund to be per-TD in practice.

LOL, yeah, a "bit".