linux-kernel - Re: [RFC PATCH v5 37/45] KVM: x86/tdp_mmu: Alloc external

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Message-ID: <aYYSIndbqLdFkaM-@google.com>
Date: Fri, 6 Feb 2026 08:09:06 -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 37/45] KVM: x86/tdp_mmu: Alloc external_spt page
 for mirror page table splitting

On Fri, Feb 06, 2026, Yan Zhao wrote:
> On Wed, Jan 28, 2026 at 05:15:09PM -0800, Sean Christopherson wrote:
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> > 
> > Enhance tdp_mmu_alloc_sp_for_split() to allocate a page table page for the
> > external page table for splitting the mirror page table.
> > 
> > Signed-off-by: Isaku Yamahata <isaku.yamahata@...el.com>
> > Co-developed-by: Yan Zhao <yan.y.zhao@...el.com>
> > Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> > [sean: use kvm_x86_ops.alloc_external_sp()]
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/kvm/mmu/tdp_mmu.c | 13 +++++++++++--
> >  1 file changed, 11 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 3b0da898824a..4f5b80f0ca03 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -1447,7 +1447,7 @@ bool kvm_tdp_mmu_wrprot_slot(struct kvm *kvm,
> >  	return spte_set;
> >  }
> >  
> > -static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(void)
> > +static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(struct tdp_iter *iter)
> >  {
> >  	struct kvm_mmu_page *sp;
> >  
> > @@ -1461,6 +1461,15 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(void)
> >  		return NULL;
> >  	}
> >  
> > +	if (is_mirror_sptep(iter->sptep)) {
> tdp_mmu_alloc_sp_for_split() is invoked in tdp_mmu_split_huge_pages_root() after
> rcu_read_unlock() is called.
> 
> So, it's incorrect to invoke is_mirror_sptep() which internally contains
> rcu_dereference(), resulting in "WARNING: suspicious RCU usage".

Ah, now I see why the previous code pass in a bool.  I don't love passing a bool,
but passing @iter is outright dangerous, so I guess this?

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index a32192c35099..4d92c0d19d7c 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1448,7 +1448,7 @@ bool kvm_tdp_mmu_wrprot_slot(struct kvm *kvm,
 }
 
 static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(struct kvm *kvm,
-                                                      struct tdp_iter *iter)
+                                                      bool is_mirror_sp)
 {
        struct kvm_mmu_page *sp;
 
@@ -1460,7 +1460,7 @@ static struct kvm_mmu_page *tdp_mmu_alloc_sp_for_split(struct kvm *kvm,
        if (!sp->spt)
                goto err_spt;
 
-       if (is_mirror_sptep(iter->sptep)) {
+       if (is_mirror_sp) {
                sp->external_spt = (void *)kvm_x86_call(alloc_external_sp)(GFP_KERNEL_ACCOUNT);
                if (!sp->external_spt)
                        goto err_external_spt;
@@ -1525,6 +1525,7 @@ static int tdp_mmu_split_huge_pages_root(struct kvm *kvm,
                                         gfn_t start, gfn_t end,
                                         int target_level, bool shared)
 {
+       const bool is_mirror_root = is_mirror_sp(root);
        struct kvm_mmu_page *sp = NULL;
        struct tdp_iter iter;
 
@@ -1557,7 +1558,7 @@ static int tdp_mmu_split_huge_pages_root(struct kvm *kvm,
                        else
                                write_unlock(&kvm->mmu_lock);
 
-                       sp = tdp_mmu_alloc_sp_for_split(kvm, &iter);
+                       sp = tdp_mmu_alloc_sp_for_split(kvm, is_mirror_root);
 
                        if (shared)
                                read_lock(&kvm->mmu_lock);