Re: [PATCH 0/5] x86: Impplement support for unaccepted memory

Kirill A. Shutemov
2021-08-10T18:40:01Z

On Tue, Aug 10, 2021 at 11:19:30AM -0700, Dave Hansen wrote:
> On 8/10/21 10:51 AM, Kirill A. Shutemov wrote:
> > On Tue, Aug 10, 2021 at 10:36:21AM -0700, Dave Hansen wrote:
> >>> The difference is going to be substantially lower once we get it optimized
> >>> properly.
> >> What does this mean? Is this future work in the kernel or somewhere in
> >> the TDX hardware/firmware which will speed things up?
> > Kernel has to be changed to accept memory in 2M and 1G chunks where
> > possible. The interface exists and described in spec, but not yet used in
> > guest kernel.
>
> From a quick scan of the spec, I only see:
>
> > 7.9.3. Page Acceptance by the Guest TD: TDG.MEM.PAGE.ACCEPT ... The guest
> > TD can accept a dynamically added 4KB page using TDG.MEM.PAGE.ACCEPT
> > with the page GPA as an input.
> Is there some other 2M/1G page-acceptance call that I'm missing?

I referred to GHCI[1], section 2.4.7. RDX=0 is 4k, RDX=1 is 2M and
RDX=2 is 1G.

Public specs have mismatches. I hope it will get sorted out soon. :/

[1] https://software.intel.com/content/dam/develop/external/us/en/documents/intel-tdx-guest-hypervisor-communication-interface.pdf

> > It would cut hypercall overhead dramatically. It makes upfront memory
> > accept more bearable and lowers latency of lazy memory accept. So I expect
> > the gap being not 20x, but like 3-5x (which is still huge).
>
> It would be nice to be able to judge the benefits of this series based
> on the final form. I guess we'll take what we can get, though.
>
> Either way, I'd still like to see some *actual* numbers for at least one
> configuration:
>
> With this series applied, userspace starts to run at X seconds
> after kernel boot. Without this series, userspace runs at Y
> seconds.

Getting absolute numbers in public for unreleased product is tricky.
I hoped to get away with ratios or percentage of difference.

--
Kirill A. Shutemov