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

Dave Hansen
2021-08-10T18:20:15Z

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?

> 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.