Re: [PATCH v12 07/22] x86/virt/tdx: Add skeleton to enable TDX on demand

[Peter Zijlstra]

On Wed, Jul 05, 2023 at 07:34:06AM -0700, Dave Hansen wrote:
> On 7/4/23 09:58, Peter Zijlstra wrote:
> > If we have concerns about allocating the PAMT array, can't we use CMA
> > for this? Allocate the whole thing at boot as CMA such that when not
> > used for TDX it can be used for regular things like userspace and
> > filecache pages?
> 
> I never thought of CMA as being super reliable.  Maybe it's improved
> over the years.
> 
> KVM also has a rather nasty habit of pinning pages, like for device
> passthrough.  I suspect that means that we'll have one of two scenarios:
> 
>  1. CMA works great, but the TDX/CMA area is unusable for KVM because
>     it's pinning all its pages and they just get moved out of the CMA
>     area immediately.  The CMA area is effectively wasted.
>  2. CMA sucks, and users get sporadic TDX failures when they wait a long
>     time to run a TDX guest after boot.  Users just work around the CMA
>     support by starting up TDX guests at boot or demanding a module
>     parameter be set.  Hacking in CMA support was a waste.
> 
> Am I just too much of a pessimist?

Well, if CMA still sucks, then that needs fixing. If CMA works, but we
have a circular fail in that KVM needs to long-term pin the PAMT pages
but long-term pin is evicted from CMA (the whole point of long-term pin,
after all), then surely we can break that cycle somehow, since in this
case the purpose of the CMA is being able to grab that memory chunk when
we needs it.

That is, either way around is just a matter of a little code, no?