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Message-ID: <aCa4jyAeZ9gFQUUQ@yzhao56-desk.sh.intel.com>
Date: Fri, 16 May 2025 12:01:19 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
CC: "pbonzini@...hat.com" <pbonzini@...hat.com>, "seanjc@...gle.com"
<seanjc@...gle.com>, "Shutemov, Kirill" <kirill.shutemov@...el.com>,
"quic_eberman@...cinc.com" <quic_eberman@...cinc.com>, "Li, Xiaoyao"
<xiaoyao.li@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Hansen,
Dave" <dave.hansen@...el.com>, "david@...hat.com" <david@...hat.com>,
"thomas.lendacky@....com" <thomas.lendacky@....com>, "tabba@...gle.com"
<tabba@...gle.com>, "Li, Zhiquan1" <zhiquan1.li@...el.com>, "Du, Fan"
<fan.du@...el.com>, "linux-kernel@...r.kernel.org"
<linux-kernel@...r.kernel.org>, "michael.roth@....com"
<michael.roth@....com>, "Weiny, Ira" <ira.weiny@...el.com>, "vbabka@...e.cz"
<vbabka@...e.cz>, "binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>,
"ackerleytng@...gle.com" <ackerleytng@...gle.com>, "Yamahata, Isaku"
<isaku.yamahata@...el.com>, "Peng, Chao P" <chao.p.peng@...el.com>,
"Annapurve, Vishal" <vannapurve@...gle.com>, "jroedel@...e.de"
<jroedel@...e.de>, "Miao, Jun" <jun.miao@...el.com>, "pgonda@...gle.com"
<pgonda@...gle.com>, "x86@...nel.org" <x86@...nel.org>
Subject: Re: [RFC PATCH 10/21] KVM: x86/mmu: Disallow page merging (huge page
adjustment) for mirror root
On Wed, May 14, 2025 at 04:15:14AM +0800, Edgecombe, Rick P wrote:
> On Thu, 2025-04-24 at 11:06 +0800, Yan Zhao wrote:
> > From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
> >
> > Disallow page merging (huge page adjustment) for mirror root by leveraging
> > the disallowed_hugepage_adjust().
> >
> > [Yan: Passing is_mirror to disallowed_hugepage_adjust()]
> >
> > Signed-off-by: Edgecombe, Rick P <rick.p.edgecombe@...el.com>
> > Signed-off-by: Yan Zhao <yan.y.zhao@...el.com>
> > ---
> > arch/x86/kvm/mmu/mmu.c | 6 +++---
> > arch/x86/kvm/mmu/mmu_internal.h | 2 +-
> > arch/x86/kvm/mmu/paging_tmpl.h | 2 +-
> > arch/x86/kvm/mmu/tdp_mmu.c | 7 ++++---
> > 4 files changed, 9 insertions(+), 8 deletions(-)
> >
> > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> > index a284dce227a0..b923deeeb62e 100644
> > --- a/arch/x86/kvm/mmu/mmu.c
> > +++ b/arch/x86/kvm/mmu/mmu.c
> > @@ -3326,13 +3326,13 @@ void kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> > fault->pfn &= ~mask;
> > }
> >
> > -void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_level)
> > +void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_level, bool is_mirror)
> > {
> > if (cur_level > PG_LEVEL_4K &&
> > cur_level == fault->goal_level &&
> > is_shadow_present_pte(spte) &&
> > !is_large_pte(spte) &&
> > - spte_to_child_sp(spte)->nx_huge_page_disallowed) {
> > + (spte_to_child_sp(spte)->nx_huge_page_disallowed || is_mirror)) {
> > /*
> > * A small SPTE exists for this pfn, but FNAME(fetch),
> > * direct_map(), or kvm_tdp_mmu_map() would like to create a
> > @@ -3363,7 +3363,7 @@ static int direct_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > * large page, as the leaf could be executable.
> > */
> > if (fault->nx_huge_page_workaround_enabled)
> > - disallowed_hugepage_adjust(fault, *it.sptep, it.level);
> > + disallowed_hugepage_adjust(fault, *it.sptep, it.level, false);
> >
> > base_gfn = gfn_round_for_level(fault->gfn, it.level);
> > if (it.level == fault->goal_level)
> > diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> > index db8f33e4de62..1c1764f46e66 100644
> > --- a/arch/x86/kvm/mmu/mmu_internal.h
> > +++ b/arch/x86/kvm/mmu/mmu_internal.h
> > @@ -411,7 +411,7 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
> > int kvm_mmu_max_mapping_level(struct kvm *kvm,
> > const struct kvm_memory_slot *slot, gfn_t gfn);
> > void kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault);
> > -void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_level);
> > +void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_level, bool is_mirror);
> >
> > void track_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);
> > void untrack_possible_nx_huge_page(struct kvm *kvm, struct kvm_mmu_page *sp);
> > diff --git a/arch/x86/kvm/mmu/paging_tmpl.h b/arch/x86/kvm/mmu/paging_tmpl.h
> > index 68e323568e95..1559182038e3 100644
> > --- a/arch/x86/kvm/mmu/paging_tmpl.h
> > +++ b/arch/x86/kvm/mmu/paging_tmpl.h
> > @@ -717,7 +717,7 @@ static int FNAME(fetch)(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault,
> > * large page, as the leaf could be executable.
> > */
> > if (fault->nx_huge_page_workaround_enabled)
> > - disallowed_hugepage_adjust(fault, *it.sptep, it.level);
> > + disallowed_hugepage_adjust(fault, *it.sptep, it.level, false);
> >
> > base_gfn = gfn_round_for_level(fault->gfn, it.level);
> > if (it.level == fault->goal_level)
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> > index 405874f4d088..8ee01277cc07 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.c
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> > @@ -1244,6 +1244,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > struct tdp_iter iter;
> > struct kvm_mmu_page *sp;
> > int ret = RET_PF_RETRY;
> > + bool is_mirror = is_mirror_sp(root);
> >
> > kvm_mmu_hugepage_adjust(vcpu, fault);
> >
> > @@ -1254,8 +1255,8 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > for_each_tdp_pte(iter, kvm, root, fault->gfn, fault->gfn + 1) {
> > int r;
> >
> > - if (fault->nx_huge_page_workaround_enabled)
> > - disallowed_hugepage_adjust(fault, iter.old_spte, iter.level);
> > + if (fault->nx_huge_page_workaround_enabled || is_mirror)
>
> Maybe we should rename nx_huge_page_workaround_enabled to something more generic
> and do the is_mirror logic in kvm_mmu_do_page_fault() when setting it. It should
> shrink the diff and centralize the logic.
Hmm, I'm reluctant to rename nx_huge_page_workaround_enabled, because
(1) Invoking disallowed_hugepage_adjust() for mirror root is to disable page
promotion for TDX private memory, so is only applied to TDP MMU.
(2) nx_huge_page_workaround_enabled is used specifically for nx huge pages.
fault->huge_page_disallowed = fault->exec && fault->nx_huge_page_workaround_enabled;
if (fault->huge_page_disallowed)
account_nx_huge_page(vcpu->kvm, sp,
fault->req_level >= it.level);
sp->nx_huge_page_disallowed = fault->huge_page_disallowed.
Affecting fault->huge_page_disallowed would impact
sp->nx_huge_page_disallowed as well and would disable huge pages entirely.
So, we still need to keep nx_huge_page_workaround_enabled.
If we introduce a new flag fault->disable_hugepage_adjust, and set it in
kvm_mmu_do_page_fault(), we would also need to invoke
tdp_mmu_get_root_for_fault() there as well.
Checking for mirror root for non-TDX VMs is not necessary, and the invocation of
tdp_mmu_get_root_for_fault() seems redundant with the one in kvm_tdp_mmu_map().
> > + disallowed_hugepage_adjust(fault, iter.old_spte, iter.level, is_mirror);
> >
> > /*
> > * If SPTE has been frozen by another thread, just give up and
> > @@ -1278,7 +1279,7 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> > */
> > sp = tdp_mmu_alloc_sp(vcpu);
> > tdp_mmu_init_child_sp(sp, &iter);
> > - if (is_mirror_sp(sp))
> > + if (is_mirror)
> > kvm_mmu_alloc_external_spt(vcpu, sp);
> >
> > sp->nx_huge_page_disallowed = fault->huge_page_disallowed;
>
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