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Message-ID: <CAAhSdy3ANMmRL6KAm5J9Amd8-1GFYpNKG_cC1gZ7Za45XsOrPA@mail.gmail.com>
Date: Fri, 9 Jan 2026 12:39:10 +0530
From: Anup Patel <anup@...infault.org>
To: liu.xuemei1@....com.cn
Cc: atish.patra@...ux.dev, alex@...ti.fr, pjw@...nel.org, palmer@...belt.com,
aou@...s.berkeley.edu, kvm@...r.kernel.org, kvm-riscv@...ts.infradead.org,
inux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] RISC-V: KVM: Transparent huge page support
On Thu, Nov 27, 2025 at 2:22 PM <liu.xuemei1@....com.cn> wrote:
>
> From: Jessica Liu <liu.xuemei1@....com.cn>
>
> Use block mapping if backed by a THP, as implemented in architectures
> like ARM and x86_64.
>
> Signed-off-by: Jessica Liu <liu.xuemei1@....com.cn>
LGTM.
Reviewed-by: Anup Patel <anup@...infault.org>
Queued this patch for Linux-6.20
Thanks,
Anup
> ---
> Changes in v4:
> - Substituted kvm_riscv_gstage_get_mapping_size with get_hva_mapping_size
>
> arch/riscv/kvm/mmu.c | 140 ++++++++++++++++++++++++++++++++++++++++
> arch/riscv/mm/pgtable.c | 2 +
> 2 files changed, 142 insertions(+)
>
> diff --git a/arch/riscv/kvm/mmu.c b/arch/riscv/kvm/mmu.c
> index 58f5f3536ffd..38816c5895fe 100644
> --- a/arch/riscv/kvm/mmu.c
> +++ b/arch/riscv/kvm/mmu.c
> @@ -302,6 +302,142 @@ bool kvm_test_age_gfn(struct kvm *kvm, struct kvm_gfn_range *range)
> return pte_young(ptep_get(ptep));
> }
>
> +static bool fault_supports_gstage_huge_mapping(struct kvm_memory_slot *memslot,
> + unsigned long hva)
> +{
> + hva_t uaddr_start, uaddr_end;
> + gpa_t gpa_start;
> + size_t size;
> +
> + size = memslot->npages * PAGE_SIZE;
> + uaddr_start = memslot->userspace_addr;
> + uaddr_end = uaddr_start + size;
> +
> + gpa_start = memslot->base_gfn << PAGE_SHIFT;
> +
> + /*
> + * Pages belonging to memslots that don't have the same alignment
> + * within a PMD for userspace and GPA cannot be mapped with g-stage
> + * PMD entries, because we'll end up mapping the wrong pages.
> + *
> + * Consider a layout like the following:
> + *
> + * memslot->userspace_addr:
> + * +-----+--------------------+--------------------+---+
> + * |abcde|fgh vs-stage block | vs-stage block tv|xyz|
> + * +-----+--------------------+--------------------+---+
> + *
> + * memslot->base_gfn << PAGE_SHIFT:
> + * +---+--------------------+--------------------+-----+
> + * |abc|def g-stage block | g-stage block |tvxyz|
> + * +---+--------------------+--------------------+-----+
> + *
> + * If we create those g-stage blocks, we'll end up with this incorrect
> + * mapping:
> + * d -> f
> + * e -> g
> + * f -> h
> + */
> + if ((gpa_start & (PMD_SIZE - 1)) != (uaddr_start & (PMD_SIZE - 1)))
> + return false;
> +
> + /*
> + * Next, let's make sure we're not trying to map anything not covered
> + * by the memslot. This means we have to prohibit block size mappings
> + * for the beginning and end of a non-block aligned and non-block sized
> + * memory slot (illustrated by the head and tail parts of the
> + * userspace view above containing pages 'abcde' and 'xyz',
> + * respectively).
> + *
> + * Note that it doesn't matter if we do the check using the
> + * userspace_addr or the base_gfn, as both are equally aligned (per
> + * the check above) and equally sized.
> + */
> + return (hva >= ALIGN(uaddr_start, PMD_SIZE)) && (hva < ALIGN_DOWN(uaddr_end, PMD_SIZE));
> +}
> +
> +static int get_hva_mapping_size(struct kvm *kvm,
> + unsigned long hva)
> +{
> + int size = PAGE_SIZE;
> + unsigned long flags;
> + pgd_t pgd;
> + p4d_t p4d;
> + pud_t pud;
> + pmd_t pmd;
> +
> + /*
> + * Disable IRQs to prevent concurrent tear down of host page tables,
> + * e.g. if the primary MMU promotes a P*D to a huge page and then frees
> + * the original page table.
> + */
> + local_irq_save(flags);
> +
> + /*
> + * Read each entry once. As above, a non-leaf entry can be promoted to
> + * a huge page _during_ this walk. Re-reading the entry could send the
> + * walk into the weeks, e.g. p*d_leaf() returns false (sees the old
> + * value) and then p*d_offset() walks into the target huge page instead
> + * of the old page table (sees the new value).
> + */
> + pgd = pgdp_get(pgd_offset(kvm->mm, hva));
> + if (pgd_none(pgd))
> + goto out;
> +
> + p4d = p4dp_get(p4d_offset(&pgd, hva));
> + if (p4d_none(p4d) || !p4d_present(p4d))
> + goto out;
> +
> + pud = pudp_get(pud_offset(&p4d, hva));
> + if (pud_none(pud) || !pud_present(pud))
> + goto out;
> +
> + if (pud_leaf(pud)) {
> + size = PUD_SIZE;
> + goto out;
> + }
> +
> + pmd = pmdp_get(pmd_offset(&pud, hva));
> + if (pmd_none(pmd) || !pmd_present(pmd))
> + goto out;
> +
> + if (pmd_leaf(pmd))
> + size = PMD_SIZE;
> +
> +out:
> + local_irq_restore(flags);
> + return size;
> +}
> +
> +static unsigned long transparent_hugepage_adjust(struct kvm *kvm,
> + struct kvm_memory_slot *memslot,
> + unsigned long hva,
> + kvm_pfn_t *hfnp, gpa_t *gpa)
> +{
> + kvm_pfn_t hfn = *hfnp;
> +
> + /*
> + * Make sure the adjustment is done only for THP pages. Also make
> + * sure that the HVA and GPA are sufficiently aligned and that the
> + * block map is contained within the memslot.
> + */
> + if (fault_supports_gstage_huge_mapping(memslot, hva)) {
> + int sz;
> +
> + sz = get_hva_mapping_size(kvm, hva);
> + if (sz < PMD_SIZE)
> + return sz;
> +
> + *gpa &= PMD_MASK;
> + hfn &= ~(PTRS_PER_PMD - 1);
> + *hfnp = hfn;
> +
> + return PMD_SIZE;
> + }
> +
> + return PAGE_SIZE;
> +}
> +
> int kvm_riscv_mmu_map(struct kvm_vcpu *vcpu, struct kvm_memory_slot *memslot,
> gpa_t gpa, unsigned long hva, bool is_write,
> struct kvm_gstage_mapping *out_map)
> @@ -395,6 +531,10 @@ int kvm_riscv_mmu_map(struct kvm_vcpu *vcpu, struct kvm_memory_slot *memslot,
> if (mmu_invalidate_retry(kvm, mmu_seq))
> goto out_unlock;
>
> + /* check if we are backed by a THP and thus use block mapping if possible */
> + if (vma_pagesize == PAGE_SIZE)
> + vma_pagesize = transparent_hugepage_adjust(kvm, memslot, hva, &hfn, &gpa);
> +
> if (writable) {
> mark_page_dirty_in_slot(kvm, memslot, gfn);
> ret = kvm_riscv_gstage_map_page(&gstage, pcache, gpa, hfn << PAGE_SHIFT,
> diff --git a/arch/riscv/mm/pgtable.c b/arch/riscv/mm/pgtable.c
> index 8b6c0a112a8d..fe776f03cc12 100644
> --- a/arch/riscv/mm/pgtable.c
> +++ b/arch/riscv/mm/pgtable.c
> @@ -49,6 +49,7 @@ pud_t *pud_offset(p4d_t *p4d, unsigned long address)
>
> return (pud_t *)p4d;
> }
> +EXPORT_SYMBOL_GPL(pud_offset);
>
> p4d_t *p4d_offset(pgd_t *pgd, unsigned long address)
> {
> @@ -57,6 +58,7 @@ p4d_t *p4d_offset(pgd_t *pgd, unsigned long address)
>
> return (p4d_t *)pgd;
> }
> +EXPORT_SYMBOL_GPL(p4d_offset);
> #endif
>
> #ifdef CONFIG_HAVE_ARCH_HUGE_VMAP
> --
> 2.27.0
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