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

[Dave Hansen]

On 7/5/23 07:57, Peter Zijlstra wrote:
> 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?

It's not a circular dependency, it's conflicting requirements.

CMA makes memory more available, but only in the face of unpinned pages.

KVM can pin lots of pages, even outside of TDX-based VMs.

So we either need to change how CMA works fundamentally or stop KVM from
pinning pages.