linux-kernel - Re: [RFC V1 0/5] x86: CVMs: Align memory conversions to 2M granularity

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Message-ID: <CAGtprH8HhCVvjARm3ZuHu04uxAENOLa--kHQhRtAfSz+4-23kg@mail.gmail.com>
Date: Thu, 1 Feb 2024 11:14:28 +0530
From: Vishal Annapurve <vannapurve@...gle.com>
To: Dave Hansen <dave.hansen@...el.com>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, pbonzini@...hat.com, 
	rientjes@...gle.com, seanjc@...gle.com, erdemaktas@...gle.com, 
	ackerleytng@...gle.com, jxgao@...gle.com, sagis@...gle.com, oupton@...gle.com, 
	peterx@...hat.com, vkuznets@...hat.com, dmatlack@...gle.com, 
	pgonda@...gle.com, michael.roth@....com, kirill@...temov.name, 
	thomas.lendacky@....com, dave.hansen@...ux.intel.com, 
	linux-coco@...ts.linux.dev, chao.p.peng@...ux.intel.com, 
	isaku.yamahata@...il.com, andrew.jones@...ux.dev, corbet@....net, hch@....de, 
	m.szyprowski@...sung.com, rostedt@...dmis.org, iommu@...ts.linux.dev
Subject: Re: [RFC V1 0/5] x86: CVMs: Align memory conversions to 2M granularity

On Wed, Jan 31, 2024 at 10:23 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> There's a bunch of code in the kernel for TDX and SEV guests.  How much
> of it uses the "CVM" nomenclature?

Right, I see that "CoCo VMs" is a more accepted term in the kernel
codebase so far, will update the references in the next version.

>
> What do you do when you need to dynamically scale up the SWIOTLB size
> and can't allocate a 2M page?  I guess you're saying here that you'd
> rather run with a too-small 2M pool than a large-enough mixed 4k/2M pool.

I am not yet certain how to ensure 2M page is always available/made
available at runtime for CoCo VMs. Few options that I can think of:
1) Reserve additional memory for CMA allocations to satisfy runtime
requests of 2M allocations.
2) Pre-reserve SWIOTLB to a safe value just like it's done today and
not rely on dynamic scaling.

Any suggestions are welcome.

>
> I also had a really hard time parsing through the problem statement and
> solution here.  I'd really suggest cleaning up the problem statement and
> more clearly differentiating the host and guest sides in the description.

Thanks for taking a look at this series. I will reword the description
in the next version. The goal basically is to ensure private and
shared memory regions are always huge page aligned.