[PATCH] kexec_core: Accept unaccepted kexec destination addresses

[Yan Zhao]

On Wed, Oct 23, 2024 at 10:44:11AM -0500, Eric W. Biederman wrote:
> "Kirill A. Shutemov" <kirill at shutemov.name> writes:
> 
> > Waiting minutes to get VM booted to shell is not feasible for most
> > deployments. Lazy is sane default to me.
> 
> Huh?
> 
> Unless my guesses about what is happening are wrong lazy is hiding
> a serious implementation deficiency.  From all hardware I have seen
> taking minutes is absolutely ridiculous.
> 
> Does writing to all of memory at full speed take minutes?  How can such
> a system be functional?
> 
> If you don't actually have to write to the pages and it is just some
> accounting function it is even more ridiculous.
> 
> 
> I had previously thought that accept_memory was the firmware call.
> Now that I see that it is just a wrapper for some hardware specific
> calls I am even more perplexed.
> 
> 
> Quite honestly what this looks like to me is that someone failed to
> enable write-combining or write-back caching when writing to memory
> when initializing the protected memory.  With the result that everything
> is moving dog slow, and people are introducing complexity left and write
> to avoid that bad implementation.
> 
> 
> Can someone please explain to me why this accept_memory stuff has to be
> slow, why it has to take minutes to do it's job.
This kexec patch is a fix to a guest(TD)'s kexce failure.

For a linux guest, the accept_memory() happens before the guest accesses a page.
It will (if the guest is a TD)
(1) trigger the host to allocate the physical page on host to map the accessed
    guest page, which might be slow with wait and sleep involved, depending on
    the memory pressure on host.
(2) initializing the protected page.

Actually most of guest memory are not accessed by guest during the guest life
cycle. accept_memory() may cause the host to commit a never-to-be-used page,
with the host physical page not even being able to get swapped out.

That's why we need a lazy accept, which does not accept_memory() until after a
page is allocated by the kernel (in alloc_page(s)).

> I would much rather spend my time figuring out how to make accept_memory
> run at a reasonable speed than to litter the kernel with more of this
> nonsense.
> 
> Eric