linux-kernel - Re: [RFC PATCH v5 20/45] KVM: x86/mmu: Allocate/free S-EPT pages using tdx_{alloc,free}_control

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Message-ID: <aYYCOiMvWfSJR1AL@google.com>
Date: Fri, 6 Feb 2026 07:01:14 -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 20/45] KVM: x86/mmu: Allocate/free S-EPT pages
 using tdx_{alloc,free}_control_page()

On Fri, Feb 06, 2026, Yan Zhao wrote:
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 18764dbc97ea..01e3e4f4baa5 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -55,7 +55,8 @@ void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm)
> >  
> >  static void tdp_mmu_free_sp(struct kvm_mmu_page *sp)
> >  {
> > -	free_page((unsigned long)sp->external_spt);
> > +	if (sp->external_spt)
> > +		kvm_x86_call(free_external_sp)((unsigned long)sp->external_spt);
> >  	free_page((unsigned long)sp->spt);
> >  	kmem_cache_free(mmu_page_header_cache, sp);
> >  }
> Strictly speaking, external_spt is not a control page. Its alloc/free are
> different from normal control pages managed by TDX's code.

Yeah, I called that out in the changelog.  I'm definitley not wedded to
tdx_{alloc,free}_control_page(), but I am very much against tdx_{alloc,free}_page().

  (arguably S-EPT pages aren't "control" pages, but they're not guest pages either)

> (1) alloc
> tdx_alloc_control_page
>   __tdx_alloc_control_page
>     __tdx_pamt_get 
>       spin_lock(&pamt_lock)   ==> under process context
>       spin_unlock(&pamt_lock)
> 
> (2) free
> tdp_mmu_free_sp_rcu_callback
>   tdp_mmu_free_sp
>     kvm_x86_call(free_external_sp)
>      tdx_free_control_page
>         __tdx_free_control_page
>           __tdx_pamt_put
>             spin_lock(&pamt_lock)   ==> under softirq context
>             spin_unlock(&pamt_lock)
> 
> So, invoking __tdx_pamt_put() in the RCU callback triggers deadlock warning
> (see the bottom for details).

Hrm.  I can think of two options.  Option #1 would be to use a raw spinlock and
disable IRQs:

diff --git a/arch/x86/virt/vmx/tdx/tdx.c b/arch/x86/virt/vmx/tdx/tdx.c
index 823ec092b4e4..6348085d7dcb 100644
--- a/arch/x86/virt/vmx/tdx/tdx.c
+++ b/arch/x86/virt/vmx/tdx/tdx.c
@@ -2246,7 +2246,7 @@ static u64 tdh_phymem_pamt_remove(u64 pfn, u64 *pamt_pa_array)
 }
 
 /* Serializes adding/removing PAMT memory */
-static DEFINE_SPINLOCK(pamt_lock);
+static DEFINE_RAW_SPINLOCK(pamt_lock);
 
 /* Bump PAMT refcount for the given page and allocate PAMT memory if needed */
 int __tdx_pamt_get(u64 pfn, struct tdx_pamt_cache *cache)
@@ -2272,7 +2272,7 @@ int __tdx_pamt_get(u64 pfn, struct tdx_pamt_cache *cache)
        if (ret)
                goto out_free;
 
-       scoped_guard(spinlock, &pamt_lock) {
+       scoped_guard(raw_spinlock_irqsave, &pamt_lock) {
                /*
                 * Lost race to other tdx_pamt_add(). Other task has already allocated
                 * PAMT memory for the HPA.
@@ -2348,7 +2348,7 @@ void __tdx_pamt_put(u64 pfn)
        if (!atomic_dec_and_test(pamt_refcount))
                return;
 
-       scoped_guard(spinlock, &pamt_lock) {
+       scoped_guard(raw_spinlock_irqsave, &pamt_lock) {
                /* Lost race with tdx_pamt_get(). */
                if (atomic_read(pamt_refcount))
                        return;

--

Option #2 would be to immediately free the page in tdx_sept_reclaim_private_sp(),
so that pages that freed via handle_removed_pt() don't defer freeing the S-EPT
page table (which, IIUC, is safe since the TDX-Module forces TLB flushes and exits).

I really, really don't like this option (if it even works).

diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
index ae7b9beb3249..4726011ad624 100644
--- a/arch/x86/kvm/vmx/tdx.c
+++ b/arch/x86/kvm/vmx/tdx.c
@@ -2014,7 +2014,15 @@ static void tdx_sept_reclaim_private_sp(struct kvm *kvm, gfn_t gfn,
         */
        if (KVM_BUG_ON(is_hkid_assigned(to_kvm_tdx(kvm)), kvm) ||
            tdx_reclaim_page(virt_to_page(sp->external_spt)))
-               sp->external_spt = NULL;
+               goto out;
+
+       /*
+        * Immediately free the control page, as the TDX subsystem doesn't
+        * support freeing pages from RCU callbacks.
+        */
+       tdx_free_control_page((unsigned long)sp->external_spt);
+out:
+       sp->external_spt = NULL;
 }
 
 void tdx_deliver_interrupt(struct kvm_lapic *apic, int delivery_mode,
--