linux-kernel - Re: [RFC PATCH v5 44/45] KVM: x86/mmu: Add support for splitting S-EPT hugepages on conversion

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Message-ID: <aYX-RpxDYrI65XRC@google.com>
Date: Fri, 6 Feb 2026 06:46:16 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Yan Zhao <yan.y.zhao@...el.com>
Cc: Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	Kiryl Shutsemau <kas@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org, 
	linux-coco@...ts.linux.dev, kvm@...r.kernel.org, 
	Kai Huang <kai.huang@...el.com>, Rick Edgecombe <rick.p.edgecombe@...el.com>, 
	Vishal Annapurve <vannapurve@...gle.com>, Ackerley Tng <ackerleytng@...gle.com>, 
	Sagi Shahar <sagis@...gle.com>, Binbin Wu <binbin.wu@...ux.intel.com>, 
	Xiaoyao Li <xiaoyao.li@...el.com>, Isaku Yamahata <isaku.yamahata@...el.com>
Subject: Re: [RFC PATCH v5 44/45] KVM: x86/mmu: Add support for splitting
 S-EPT hugepages on conversion

On Fri, Feb 06, 2026, Yan Zhao wrote:
> On Wed, Jan 28, 2026 at 05:15:16PM -0800, Sean Christopherson wrote:
> > Add support for splitting S-EPT hugepages in preparation for converting a
> > subset of a hugepage to be shared, as KVM must precisely zap/remove S-EPT
> > entries to avoid clobbering guest memory (the lifetime of guest private
> > memory is tied to the S-EPT).  I.e. KVM needs to first split a hugepage so
> > that only the to-be-converted small pages can be zapped.
> > 
> > To avoid unnecessary work, e.g. if only the tail/end page of massive region
> > isn't aligned to the conversion, explicitly detect unaligned head and tail
> > pages relative to the max page size support by KVM, i.e. head/tail pages
> > that will undergo partial conversion.
> > 
> > To support splitting an S-EPT hugepage without a vCPU, add a per-VM PAMT
> > cache, along with a mutex to guard the cache.  Using a mutex, e.g. versus
> > a spinlock, is important at it allows KVM to allocate memory *without*
> > dropping the lock, i.e. so that the PAMT cache can be topped-up as needed
> > without needed to juggle arch.tdp_mmu_external_cache_lock.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/include/asm/kvm_host.h |  8 +++-
> >  arch/x86/kvm/mmu/mmu.c          |  2 +-
> >  arch/x86/kvm/mmu/tdp_mmu.c      | 72 +++++++++++++++++++++++++++++++--
> >  arch/x86/kvm/vmx/tdx.c          | 34 +++++++++++++---
> >  arch/x86/kvm/vmx/tdx.h          |  2 +
> >  5 files changed, 107 insertions(+), 11 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> > index 385f1cf32d70..54dea90a53dc 100644
> > --- a/arch/x86/include/asm/kvm_host.h
> > +++ b/arch/x86/include/asm/kvm_host.h
> > @@ -1563,6 +1563,12 @@ struct kvm_arch {
> >  	 * the code to do so.
> >  	 */
> >  	spinlock_t tdp_mmu_pages_lock;
> > +
> > +	/*
> > +	 * Protect the per-VM cache of pre-allocate pages used to populate the
> > +	 * Dynamic PAMT when splitting S-EPT huge pages without a vCPU.
> > +	 */
> > +	struct mutex tdp_mmu_external_cache_lock;
> Missing "spin_lock_init(&kvm->arch.tdp_mmu_external_cache_lock);" in
> kvm_mmu_init_tdp_mmu().
> 
> Will check the patch you replied next week.

It has the same bug.  FWIW, I found and fixed the bug on our internal branch, but I
either missed the fixup when synchronizing back to the upstream branch, or I found
the issue after posting.

@@ -634,6 +642,7 @@ int tdx_vm_init(struct kvm *kvm)
        struct kvm_tdx *kvm_tdx = to_kvm_tdx(kvm);
 
        tdx_init_pamt_cache(&kvm_tdx->pamt_cache);
+       mutex_init(&kvm_tdx->pamt_cache_lock);
 
        kvm->arch.has_protected_state = true;
        /*