linux-kernel - Re: [PATCH] kexec_core: Accept unaccepted kexec destination addresses

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Message-ID: <ihzvi5pwn5hrn4ky2ehjqztjxoixaiaby4igmeihqfehy2vrii@tsg6j5qvmyrm>
Date: Fri, 25 Oct 2024 16:56:41 +0300
From: "Kirill A. Shutemov" <kirill@...temov.name>
To: "Eric W. Biederman" <ebiederm@...ssion.com>
Cc: Yan Zhao <yan.y.zhao@...el.com>, kexec@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, linux-coco@...ts.linux.dev, x86@...nel.org, 
	rick.p.edgecombe@...el.com, kirill.shutemov@...ux.intel.com
Subject: Re: [PATCH] kexec_core: Accept unaccepted kexec destination addresses

On Wed, Oct 23, 2024 at 10:44:11AM -0500, Eric W. Biederman wrote:
> "Kirill A. Shutemov" <kirill@...temov.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?

It is not only memory write (to encrypt the memory), but also TDCALL which
is TD-exit on every page. That is costly in TDX case.

On single vCPU it takes about a minute to accept 90GiB of memory.

It improves a bit with number of vCPUs. It is 40 seconds with 4 vCPU, but
it doesn't scale past that in my setup.

But it is all rather pathological: VMM doesn't support huge pages yet and
all memory is accepted in 4K chunks. Bringing 2M support would cut number
of TDCALLs by 512.

Once memory accepted, memory access cost is comparable to bare metal minus
usual virtualisation tax on page walk.

I don't know what the picture looks like in AMD case.
j
> 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.

It is hypercall basically. The feature is only used in guests so far.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov