Re: [PATCH] kexec_core: Accept unaccepted kexec destination addresses

[Baoquan He <bhe@redhat.com>]

On 10/24/24 at 08:15am, Yan Zhao wrote:
> On Wed, Oct 23, 2024 at 10:44:11AM -0500, Eric W. Biederman wrote:
> > "Kirill A. Shutemov" <kirill@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.

So this sounds to me more like a business requirement on cloud platform,
e.g if one customer books a guest instance with 60G memory, while the
customer actually always only cost 20G memory at most. Then the 40G memory
can be saved to reduce pressure for host. I could be shallow, just a wild
guess.

If my guess is right, at least those cloud service providers must like this
accept_memory feature very much.

> 
> 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)).

By the way, I have two questions, maybe very shallow.

1) why can't we only find those already accepted memory to put kexec
kernel/initrd/bootparam/purgatory?

2) why can't we accept memory for (kernel, boot params/cmdline/initrd)
in 2nd kernel? Surely this purgatory still need be accepted in 1st kernel.
Sorry, I just read accept_memory() code, haven't gone through x86 boot
code flow.

Thanks
Baoquan