linux-kernel - Re: [RFC PATCH] KVM: TDX: Decouple TDX init mem region from kvm_gmem

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Message-ID: <aJJimk8FnfnYaZ2j@google.com>
Date: Tue, 5 Aug 2025 12:59:22 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Vishal Annapurve <vannapurve@...gle.com>
Cc: Ira Weiny <ira.weiny@...el.com>, Yan Zhao <yan.y.zhao@...el.com>, 
	Michael Roth <michael.roth@....com>, pbonzini@...hat.com, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, rick.p.edgecombe@...el.com, kai.huang@...el.com, 
	adrian.hunter@...el.com, reinette.chatre@...el.com, xiaoyao.li@...el.com, 
	tony.lindgren@...el.com, binbin.wu@...ux.intel.com, dmatlack@...gle.com, 
	isaku.yamahata@...el.com, david@...hat.com, ackerleytng@...gle.com, 
	tabba@...gle.com, chao.p.peng@...el.com
Subject: Re: [RFC PATCH] KVM: TDX: Decouple TDX init mem region from kvm_gmem_populate()

On Mon, Aug 04, 2025, Vishal Annapurve wrote:
> On Mon, Aug 4, 2025 at 5:22 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > : 4) For SNP, if src != null, make the target pfn to be shared, copy
> > : contents and then make the target pfn back to private.
> >
> > Copying from userspace under spinlock (rwlock) is illegal, as accessing userspace
> > memory might_fault() and thus might_sleep().
> 
> I would think that a combination of get_user_pages() and
> kmap_local_pfn() will prevent this situation of might_fault().

Yes, but if SNP is using get_user_pages(), then it looks an awful lot like the
TDX flow, at which point isn't that an argument for keeping populate()?

> Memory population in my opinion is best solved either by users asserting
> ownership of the memory and writing to it directly or by using guest_memfd
> (to be) exposed APIs to populate memory ranges given a source buffer. IMO
> kvm_gmem_populate() is doing something different than both of these options.

In a perfect world, yes, guest_memfd would provide a clean, well-defined API
without needing a complicated dance between vendor code and guest_memfd.  But,
sadly, the world of CoCo is anything but perfect.  It's not KVM's fault that
every vendor came up with a different CoCo architecture.  I.e. we can't "fix"
the underlying issue of SNP and TDX having significantly different ways for
initializing private memory.

What we can do is shift as much code to common KVM as possible, e.g. to minimize
maintenance costs, reduce boilerplate and/or copy+paste code, provide a consistent
ABI, etc.  Those things always need to be balanced against overall complexity, but
IMO providing a vendor callback doesn't add anywhere near enough complexity to
justify open coding the same concept in every vendor implementation.