linux-kernel - Re: [RFC PATCH 09/21] KVM: TDX: Enable 2MB mapping size after TD is RUNNABLE

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Message-ID: <aFWNLZQ7pqQahdEh@google.com>
Date: Fri, 20 Jun 2025 09:32:45 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Kirill Shutemov <kirill.shutemov@...el.com>
Cc: Rick P Edgecombe <rick.p.edgecombe@...el.com>, Yan Y Zhao <yan.y.zhao@...el.com>, 
	Fan Du <fan.du@...el.com>, Xiaoyao Li <xiaoyao.li@...el.com>, Kai Huang <kai.huang@...el.com>, 
	"quic_eberman@...cinc.com" <quic_eberman@...cinc.com>, Dave Hansen <dave.hansen@...el.com>, 
	"david@...hat.com" <david@...hat.com>, "thomas.lendacky@....com" <thomas.lendacky@....com>, 
	"vbabka@...e.cz" <vbabka@...e.cz>, Zhiquan1 Li <zhiquan1.li@...el.com>, 
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "michael.roth@....com" <michael.roth@....com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Chao P Peng <chao.p.peng@...el.com>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, Ira Weiny <ira.weiny@...el.com>, 
	Isaku Yamahata <isaku.yamahata@...el.com>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>, 
	"ackerleytng@...gle.com" <ackerleytng@...gle.com>, Vishal Annapurve <vannapurve@...gle.com>, 
	"tabba@...gle.com" <tabba@...gle.com>, "jroedel@...e.de" <jroedel@...e.de>, Jun Miao <jun.miao@...el.com>, 
	"pgonda@...gle.com" <pgonda@...gle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC PATCH 09/21] KVM: TDX: Enable 2MB mapping size after TD is RUNNABLE

On Wed, Jun 18, 2025, Kirill Shutemov wrote:
> On Wed, Jun 18, 2025 at 04:22:59AM +0300, Edgecombe, Rick P wrote:
> > On Tue, 2025-06-17 at 08:52 +0800, Yan Zhao wrote:
> > > > hopefully is just handling accepting a whole range that is not 2MB aligned.
> > > > But
> > > > I think we need to verify this more.
> > > Ok.
> > 
> > In Linux guest if a memory region is not 2MB aligned the guest will accept the

What is a "memory region" in this context?  An e820 region?  Something else?

> > ends at 4k size. If a memory region is identical to a memslot range this will be
> > fine. KVM will map the ends at 4k because it won't let huge pages span a
> > memslot. But if several memory regions are not 2MB aligned and are covered by
> > one large memslot, the accept will fail on the 4k ends under this proposal. I
> > don't know if this is a common configuration, but to cover it in the TDX guest
> > may not be trivial.
> > 
> > So I think this will only work if guests can reasonably "merge" all of the
> > adjacent accepts. Or of we declare a bunch of memory/memslot layouts illegal.
> > 
> > Kirill, how difficult would it be for TDX Linux guest to merge all 2MB adjacent
> > accepts?
> 
> Hm. What do you mean by merging?
> 
> Kernel only accepts <4k during early boot -- in EFI stub. The bitmap we
> use to track unaccepted memory tracks the status in 2M granularity and
> all later accept requests will be issues on 2M pages with fallback to 4k.
> 
> -- 
>   Kiryl Shutsemau / Kirill A. Shutemov