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Message-ID: <aWdhkpXRLEek7AnL@google.com>
Date: Wed, 14 Jan 2026 09:27:46 +0000
From: Vincent Donnefort <vdonnefort@...gle.com>
To: Petteri Kangaslampi <pekangas@...gle.com>
Cc: kvmarm@...ts.linux.dev, Marc Zyngier <maz@...nel.org>,
Oliver Upton <oliver.upton@...ux.dev>,
Joey Gouly <joey.gouly@....com>,
Suzuki K Poulose <suzuki.poulose@....com>,
Zenghui Yu <yuzenghui@...wei.com>,
Catalin Marinas <catalin.marinas@....com>,
Will Deacon <will@...nel.org>, linux-arm-kernel@...ts.infradead.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/1] KVM: arm64: Calculate hyp VA size only once
On Tue, Jan 13, 2026 at 07:44:09PM +0000, Petteri Kangaslampi wrote:
> Calculate the hypervisor's VA size only once to maintain consistency
> between the memory layout and MMU initialization logic. Previously the
> two would be inconsistent when the kernel is configured for less than
> IDMAP_VA_BITS of VA space.
>
> Signed-off-by: Petteri Kangaslampi <pekangas@...gle.com>
Tested-by: Vincent Donnefort <vdonnefort@...gle.com>
> ---
> arch/arm64/include/asm/kvm_mmu.h | 3 ++-
> arch/arm64/kvm/arm.c | 4 ++--
> arch/arm64/kvm/mmu.c | 28 ++++-----------------------
> arch/arm64/kvm/va_layout.c | 33 +++++++++++++++++++++++++++-----
> 4 files changed, 36 insertions(+), 32 deletions(-)
>
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index 2dc5e6e742bb..d968aca0461a 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -103,6 +103,7 @@ alternative_cb_end
> void kvm_update_va_mask(struct alt_instr *alt,
> __le32 *origptr, __le32 *updptr, int nr_inst);
> void kvm_compute_layout(void);
> +u32 kvm_hyp_va_bits(void);
> void kvm_apply_hyp_relocations(void);
>
> #define __hyp_pa(x) (((phys_addr_t)(x)) + hyp_physvirt_offset)
> @@ -185,7 +186,7 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu);
>
> phys_addr_t kvm_mmu_get_httbr(void);
> phys_addr_t kvm_get_idmap_vector(void);
> -int __init kvm_mmu_init(u32 *hyp_va_bits);
> +int __init kvm_mmu_init(u32 hyp_va_bits);
>
> static inline void *__kvm_vector_slot2addr(void *base,
> enum arm64_hyp_spectre_vector slot)
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 4f80da0c0d1d..4703f0e15102 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -2568,7 +2568,7 @@ static void pkvm_hyp_init_ptrauth(void)
> /* Inits Hyp-mode on all online CPUs */
> static int __init init_hyp_mode(void)
> {
> - u32 hyp_va_bits;
> + u32 hyp_va_bits = kvm_hyp_va_bits();
> int cpu;
> int err = -ENOMEM;
>
> @@ -2582,7 +2582,7 @@ static int __init init_hyp_mode(void)
> /*
> * Allocate Hyp PGD and setup Hyp identity mapping
> */
> - err = kvm_mmu_init(&hyp_va_bits);
> + err = kvm_mmu_init(hyp_va_bits);
> if (err)
> goto out_err;
>
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 48d7c372a4cd..d5a506c99f73 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -2284,11 +2284,9 @@ static struct kvm_pgtable_mm_ops kvm_hyp_mm_ops = {
> .virt_to_phys = kvm_host_pa,
> };
>
> -int __init kvm_mmu_init(u32 *hyp_va_bits)
> +int __init kvm_mmu_init(u32 hyp_va_bits)
> {
> int err;
> - u32 idmap_bits;
> - u32 kernel_bits;
>
> hyp_idmap_start = __pa_symbol(__hyp_idmap_text_start);
> hyp_idmap_start = ALIGN_DOWN(hyp_idmap_start, PAGE_SIZE);
> @@ -2302,25 +2300,7 @@ int __init kvm_mmu_init(u32 *hyp_va_bits)
> */
> BUG_ON((hyp_idmap_start ^ (hyp_idmap_end - 1)) & PAGE_MASK);
>
> - /*
> - * The ID map is always configured for 48 bits of translation, which
> - * may be fewer than the number of VA bits used by the regular kernel
> - * stage 1, when VA_BITS=52.
> - *
> - * At EL2, there is only one TTBR register, and we can't switch between
> - * translation tables *and* update TCR_EL2.T0SZ at the same time. Bottom
> - * line: we need to use the extended range with *both* our translation
> - * tables.
> - *
> - * So use the maximum of the idmap VA bits and the regular kernel stage
> - * 1 VA bits to assure that the hypervisor can both ID map its code page
> - * and map any kernel memory.
> - */
> - idmap_bits = IDMAP_VA_BITS;
> - kernel_bits = vabits_actual;
> - *hyp_va_bits = max(idmap_bits, kernel_bits);
> -
> - kvm_debug("Using %u-bit virtual addresses at EL2\n", *hyp_va_bits);
> + kvm_debug("Using %u-bit virtual addresses at EL2\n", hyp_va_bits);
> kvm_debug("IDMAP page: %lx\n", hyp_idmap_start);
> kvm_debug("HYP VA range: %lx:%lx\n",
> kern_hyp_va(PAGE_OFFSET),
> @@ -2345,7 +2325,7 @@ int __init kvm_mmu_init(u32 *hyp_va_bits)
> goto out;
> }
>
> - err = kvm_pgtable_hyp_init(hyp_pgtable, *hyp_va_bits, &kvm_hyp_mm_ops);
> + err = kvm_pgtable_hyp_init(hyp_pgtable, hyp_va_bits, &kvm_hyp_mm_ops);
> if (err)
> goto out_free_pgtable;
>
> @@ -2354,7 +2334,7 @@ int __init kvm_mmu_init(u32 *hyp_va_bits)
> goto out_destroy_pgtable;
>
> io_map_base = hyp_idmap_start;
> - __hyp_va_bits = *hyp_va_bits;
> + __hyp_va_bits = hyp_va_bits;
> return 0;
>
> out_destroy_pgtable:
> diff --git a/arch/arm64/kvm/va_layout.c b/arch/arm64/kvm/va_layout.c
> index 91b22a014610..2346f9435a71 100644
> --- a/arch/arm64/kvm/va_layout.c
> +++ b/arch/arm64/kvm/va_layout.c
> @@ -46,9 +46,31 @@ static void init_hyp_physvirt_offset(void)
> hyp_physvirt_offset = (s64)__pa(kern_va) - (s64)hyp_va;
> }
>
> +/*
> + * Calculate the actual VA size used by the hypervisor
> + */
> +__init u32 kvm_hyp_va_bits(void)
> +{
> + /*
> + * The ID map is always configured for 48 bits of translation, which may
> + * be different from the number of VA bits used by the regular kernel
> + * stage 1.
> + *
> + * At EL2, there is only one TTBR register, and we can't switch between
> + * translation tables *and* update TCR_EL2.T0SZ at the same time. Bottom
> + * line: we need to use the extended range with *both* our translation
> + * tables.
> + *
> + * So use the maximum of the idmap VA bits and the regular kernel stage
> + * 1 VA bits as the hypervisor VA size to assure that the hypervisor can
> + * both ID map its code page and map any kernel memory.
> + */
> + return max(IDMAP_VA_BITS, vabits_actual);
> +}
> +
> /*
> * We want to generate a hyp VA with the following format (with V ==
> - * vabits_actual):
> + * hypervisor VA bits):
> *
> * 63 ... V | V-1 | V-2 .. tag_lsb | tag_lsb - 1 .. 0
> * ---------------------------------------------------------
> @@ -61,10 +83,11 @@ __init void kvm_compute_layout(void)
> {
> phys_addr_t idmap_addr = __pa_symbol(__hyp_idmap_text_start);
> u64 hyp_va_msb;
> + u32 hyp_va_bits = kvm_hyp_va_bits();
>
> /* Where is my RAM region? */
> - hyp_va_msb = idmap_addr & BIT(vabits_actual - 1);
> - hyp_va_msb ^= BIT(vabits_actual - 1);
> + hyp_va_msb = idmap_addr & BIT(hyp_va_bits - 1);
> + hyp_va_msb ^= BIT(hyp_va_bits - 1);
>
> tag_lsb = fls64((u64)phys_to_virt(memblock_start_of_DRAM()) ^
> (u64)(high_memory - 1));
> @@ -72,9 +95,9 @@ __init void kvm_compute_layout(void)
> va_mask = GENMASK_ULL(tag_lsb - 1, 0);
> tag_val = hyp_va_msb;
>
> - if (IS_ENABLED(CONFIG_RANDOMIZE_BASE) && tag_lsb != (vabits_actual - 1)) {
> + if (IS_ENABLED(CONFIG_RANDOMIZE_BASE) && tag_lsb != (hyp_va_bits - 1)) {
> /* We have some free bits to insert a random tag. */
> - tag_val |= get_random_long() & GENMASK_ULL(vabits_actual - 2, tag_lsb);
> + tag_val |= get_random_long() & GENMASK_ULL(hyp_va_bits - 2, tag_lsb);
> }
> tag_val >>= tag_lsb;
>
> --
> 2.52.0.457.g6b5491de43-goog
>
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