() On Tue, May 10, 2022 at 5:04 AM David Matlack <dmatlack@xxxxxxxxxx> wrote: > > On Sat, May 7, 2022 at 1:28 AM Lai Jiangshan <jiangshanlai@xxxxxxxxx> wrote: > > > > > > > > On 2022/4/23 05:05, David Matlack wrote: > > > Instead of computing the shadow page role from scratch for every new > > > page, derive most of the information from the parent shadow page. This > > > avoids redundant calculations and reduces the number of parameters to > > > kvm_mmu_get_page(). > > > > > > Preemptively split out the role calculation to a separate function for > > > use in a following commit. > > > > > > No functional change intended. > > > > > > Reviewed-by: Peter Xu <peterx@xxxxxxxxxx> > > > Signed-off-by: David Matlack <dmatlack@xxxxxxxxxx> > > > --- > > > arch/x86/kvm/mmu/mmu.c | 96 +++++++++++++++++++++++----------- > > > arch/x86/kvm/mmu/paging_tmpl.h | 9 ++-- > > > 2 files changed, 71 insertions(+), 34 deletions(-) > > > > > > diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c > > > index dc20eccd6a77..4249a771818b 100644 > > > --- a/arch/x86/kvm/mmu/mmu.c > > > +++ b/arch/x86/kvm/mmu/mmu.c > > > @@ -2021,31 +2021,15 @@ static void clear_sp_write_flooding_count(u64 *spte) > > > __clear_sp_write_flooding_count(sptep_to_sp(spte)); > > > } > > > > > > -static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu, > > > - gfn_t gfn, > > > - gva_t gaddr, > > > - unsigned level, > > > - bool direct, > > > - unsigned int access) > > > +static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu, gfn_t gfn, > > > + union kvm_mmu_page_role role) > > > { > > > - union kvm_mmu_page_role role; > > > struct hlist_head *sp_list; > > > - unsigned quadrant; > > > struct kvm_mmu_page *sp; > > > int ret; > > > int collisions = 0; > > > LIST_HEAD(invalid_list); > > > > > > - role = vcpu->arch.mmu->root_role; > > > - role.level = level; > > > - role.direct = direct; > > > - role.access = access; > > > - if (role.has_4_byte_gpte) { > > > - quadrant = gaddr >> (PAGE_SHIFT + (PT64_PT_BITS * level)); > > > - quadrant &= (1 << ((PT32_PT_BITS - PT64_PT_BITS) * level)) - 1; > > > - role.quadrant = quadrant; > > > - } > > > > > > I don't think replacing it with kvm_mmu_child_role() can reduce any calculations. > > > > role.level, role.direct, role.access and role.quadrant still need to be > > calculated. And the old code is still in mmu_alloc_root(). > > You are correct. Instead of saying "less calculations" I should have > said "eliminates the dependency on vcpu->arch.mmu->root_role". > > > > > I think kvm_mmu_get_shadow_page() can keep the those parameters and > > kvm_mmu_child_role() is only introduced for nested_mmu_get_sp_for_split(). > > > > Both kvm_mmu_get_shadow_page() and nested_mmu_get_sp_for_split() call > > __kvm_mmu_get_shadow_page() which uses role as a parameter. > > I agree this would work, but I think the end result would be more > difficult to read. > > The way I've implemented it there are two ways the SP roles are calculated: > > 1. For root SPs: Derive the role from vCPU root_role and caller-provided inputs. > 2. For child SPs: Derive the role from parent SP and caller-provided inputs. > > Your proposal would still have two ways to calculate the SP role, but > split along a different dimension: > > 1. For vCPUs creating SPs: Derive the role from vCPU root_role and > caller-provided inputs. > 2. For Eager Page Splitting creating SPs: Derive the role from parent > SP and caller-provided inputs. > > With your proposal, it is less obvious that eager page splitting is > going to end up with the correct role. Whereas if we use the same > calculation for all child SPs, it is immediately obvious. In this patchset, there are still two ways to calculate the SP role including the one in mmu_alloc_root() which I dislike. The old code is just moved into mmu_alloc_root() even kvm_mmu_child_role() is introduced. > > > > > > > > > > - > > > sp_list = &vcpu->kvm->arch.mmu_page_hash[kvm_page_table_hashfn(gfn)]; > > > for_each_valid_sp(vcpu->kvm, sp, sp_list) { > > > if (sp->gfn != gfn) { > > > @@ -2063,7 +2047,7 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu, > > > * Unsync pages must not be left as is, because the new > > > * upper-level page will be write-protected. > > > */ > > > - if (level > PG_LEVEL_4K && sp->unsync) > > > + if (role.level > PG_LEVEL_4K && sp->unsync) > > > kvm_mmu_prepare_zap_page(vcpu->kvm, sp, > > > &invalid_list); > > > continue; > > > @@ -2104,14 +2088,14 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu, > > > > > > ++vcpu->kvm->stat.mmu_cache_miss; > > > > > > - sp = kvm_mmu_alloc_page(vcpu, direct); > > > + sp = kvm_mmu_alloc_page(vcpu, role.direct); > > > > > > sp->gfn = gfn; > > > sp->role = role; > > > hlist_add_head(&sp->hash_link, sp_list); > > > - if (!direct) { > > > + if (!role.direct) { > > > account_shadowed(vcpu->kvm, sp); > > > - if (level == PG_LEVEL_4K && kvm_vcpu_write_protect_gfn(vcpu, gfn)) > > > + if (role.level == PG_LEVEL_4K && kvm_vcpu_write_protect_gfn(vcpu, gfn)) > > > kvm_flush_remote_tlbs_with_address(vcpu->kvm, gfn, 1); > > > } > > > trace_kvm_mmu_get_page(sp, true); > > > @@ -2123,6 +2107,51 @@ static struct kvm_mmu_page *kvm_mmu_get_page(struct kvm_vcpu *vcpu, > > > return sp; > > > } > > > > > > +static union kvm_mmu_page_role kvm_mmu_child_role(u64 *sptep, bool direct, u32 access) > > > +{ > > > + struct kvm_mmu_page *parent_sp = sptep_to_sp(sptep); > > > + union kvm_mmu_page_role role; > > > + > > > + role = parent_sp->role; > > > + role.level--; > > > + role.access = access; > > > + role.direct = direct; > > > + > > > + /* > > > + * If the guest has 4-byte PTEs then that means it's using 32-bit, > > > + * 2-level, non-PAE paging. KVM shadows such guests using 4 PAE page > > > + * directories, each mapping 1/4 of the guest's linear address space > > > + * (1GiB). The shadow pages for those 4 page directories are > > > + * pre-allocated and assigned a separate quadrant in their role. > > > > > > It is not going to be true in patchset: > > [PATCH V2 0/7] KVM: X86/MMU: Use one-off special shadow page for special roots > > > > https://lore.kernel.org/lkml/20220503150735.32723-1-jiangshanlai@xxxxxxxxx/ > > > > The shadow pages for those 4 page directories are also allocated on demand. > > Ack. I can even just drop this sentence in v5, it's just background information. No, if one-off special shadow pages are used. kvm_mmu_child_role() should be: + if (role.has_4_byte_gpte) { + if (role.level == PG_LEVEL_4K) + role.quadrant = (sptep - parent_sp->spt) % 2; + if (role.level == PG_LEVEL_2M) + role.quadrant = (sptep - parent_sp->spt) % 4; + } And if one-off special shadow pages are merged first. You don't need any calculation in mmu_alloc_root(), you can just directly use sp = kvm_mmu_get_page(vcpu, gfn, vcpu->arch.mmu->root_role); because vcpu->arch.mmu->root_role is always the real role of the root sp no matter if it is a normall root sp or an one-off special sp. I hope you will pardon me for my touting my patchset and asking people to review them in your threads. Thanks Lai