[PATCH v2 1/6] KVM: arm64: VM exit to userspace to handle SEA
Jiaqi Yan
jiaqiyan at google.com
Sat Jul 19 14:24:45 PDT 2025
On Sat, Jul 12, 2025 at 12:57 PM Oliver Upton <oliver.upton at linux.dev> wrote:
>
> On Fri, Jul 11, 2025 at 04:59:11PM -0700, Jiaqi Yan wrote:
> > > - Add some detail about FEAT_RAS where we may still exit to userspace
> > > for host-controlled memory, as we cannot differentiate between a
> > > stage-1 or stage-2 TTW SEA when taken on the descriptor PA
> >
> > Ah, IIUC, you are saying even if the FSC code tells fault is on TTW
> > (esr_fsc_is_secc_ttw or esr_fsc_is_sea_ttw), it can either be guest
> > stage-1's or stage-2's descriptor PA, and we can tell which from
> > which.
> >
> > However, if ESR_ELx_S1PTW is set, we can tell this is a sub-case of
> > stage-2 descriptor PA, their usage is for stage-1 PTW but they are
> > stage-2 memory.
> >
> > Is my current understanding right?
>
> Yep, that's exactly what I'm getting at. As you note, stage-2 aborts
> during a stage-1 walk are sufficiently described, but not much else.
Got it, thanks!
>
> > > +/*
> > > + * Returns true if the SEA should be handled locally within KVM if the abort is
> > > + * caused by a kernel memory allocation (e.g. stage-2 table memory).
> > > + */
> > > +static bool host_owns_sea(struct kvm_vcpu *vcpu, u64 esr)
> > > +{
> > > + /*
> > > + * Without FEAT_RAS HCR_EL2.TEA is RES0, meaning any external abort
> > > + * taken from a guest EL to EL2 is due to a host-imposed access (e.g.
> > > + * stage-2 PTW).
> > > + */
> > > + if (!cpus_have_final_cap(ARM64_HAS_RAS_EXTN))
> > > + return true;
> > > +
> > > + /* KVM owns the VNCR when the vCPU isn't in a nested context. */
> > > + if (is_hyp_ctxt(vcpu) && (esr & ESR_ELx_VNCR))
> > > + return true;
> > > +
> > > + /*
> > > + * Determining if an external abort during a table walk happened at
> > > + * stage-2 is only possible with S1PTW is set. Otherwise, since KVM
> > > + * sets HCR_EL2.TEA, SEAs due to a stage-1 walk (i.e. accessing the PA
> > > + * of the stage-1 descriptor) can reach here and are reported with a
> > > + * TTW ESR value.
> > > + */
> > > + return esr_fsc_is_sea_ttw(esr) && (esr & ESR_ELx_S1PTW);
> >
> > Should we include esr_fsc_is_secc_ttw? like
> > (esr_fsc_is_sea_ttw(esr) || esr_fsc_is_secc_ttw(esr)) && (esr & ESR_ELx_S1PTW)
>
> Parity / ECC errors are not permitted if FEAT_RAS is implemented (which
> is tested for up front).
Ah, thanks for pointing this out.
>
> > > +}
> > > +
> > > int kvm_handle_guest_sea(struct kvm_vcpu *vcpu)
> > > {
> > > + u64 esr = kvm_vcpu_get_esr(vcpu);
> > > + struct kvm_run *run = vcpu->run;
> > > + struct kvm *kvm = vcpu->kvm;
> > > + u64 esr_mask = ESR_ELx_EC_MASK |
> > > + ESR_ELx_FnV |
> > > + ESR_ELx_EA |
> > > + ESR_ELx_CM |
> > > + ESR_ELx_WNR |
> > > + ESR_ELx_FSC;
> >
> > Do you (and why) exclude ESR_ELx_IL on purpose?
>
> Unintended :)
Will add into my patch.
>
> > BTW, if my previous statement about TTW SEA is correct, then I also
> > understand why we need to explicitly exclude ESR_ELx_S1PTW.
>
> Right, we shouldn't be exposing genuine stage-2 external aborts to userspace.
>
> > > + u64 ipa;
> > > +
> > > +
> > > /*
> > > * Give APEI the opportunity to claim the abort before handling it
> > > * within KVM. apei_claim_sea() expects to be called with IRQs
> > > @@ -1824,7 +1864,32 @@ int kvm_handle_guest_sea(struct kvm_vcpu *vcpu)
> > > if (apei_claim_sea(NULL) == 0)
> >
> > I assume kvm should still lockdep_assert_irqs_enabled(), right? That
> > is, a WARN_ON_ONCE is still useful in case?
>
> Ah, this is diffed against my VNCR prefix which has this context. Yes, I
> want to preserve the lockdep assertion.
Thanks for sharing the patch! Should I wait for you to send and queue
to kvmarm/next and rebase my v3 to it? Or should I insert it into my
v3 patch series with you as the commit author, and Signed-off-by you?
BTW, while I am working on v3, I think it is probably better to
decouple the current patchset into two. The first one for
KVM_EXIT_ARM_SEA, and the second one for injecting (D|I)ABT with
user-supplemented esr. This way may help KVM_EXIT_ARM_SEA, the more
important feature, get reviewed and accepted sooner. I will send out a
separate patchset for enhancing the guest SEA injection.
>
>
> From eb63dbf07b3d1f42b059f5c94abd147d195299c8 Mon Sep 17 00:00:00 2001
> From: Oliver Upton <oliver.upton at linux.dev>
> Date: Thu, 10 Jul 2025 17:14:51 -0700
> Subject: [PATCH] KVM: arm64: nv: Handle SEAs due to VNCR redirection
>
> Signed-off-by: Oliver Upton <oliver.upton at linux.dev>
> ---
> arch/arm64/include/asm/kvm_mmu.h | 1 +
> arch/arm64/include/asm/kvm_ras.h | 25 -------------------------
> arch/arm64/kvm/mmu.c | 30 ++++++++++++++++++------------
> arch/arm64/kvm/nested.c | 3 +++
> 4 files changed, 22 insertions(+), 37 deletions(-)
> delete mode 100644 arch/arm64/include/asm/kvm_ras.h
>
> diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> index ae563ebd6aee..e4069f2ce642 100644
> --- a/arch/arm64/include/asm/kvm_mmu.h
> +++ b/arch/arm64/include/asm/kvm_mmu.h
> @@ -180,6 +180,7 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu);
> int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa,
> phys_addr_t pa, unsigned long size, bool writable);
>
> +int kvm_handle_guest_sea(struct kvm_vcpu *vcpu);
> int kvm_handle_guest_abort(struct kvm_vcpu *vcpu);
>
> phys_addr_t kvm_mmu_get_httbr(void);
> diff --git a/arch/arm64/include/asm/kvm_ras.h b/arch/arm64/include/asm/kvm_ras.h
> deleted file mode 100644
> index 9398ade632aa..000000000000
> --- a/arch/arm64/include/asm/kvm_ras.h
> +++ /dev/null
> @@ -1,25 +0,0 @@
> -/* SPDX-License-Identifier: GPL-2.0 */
> -/* Copyright (C) 2018 - Arm Ltd */
> -
> -#ifndef __ARM64_KVM_RAS_H__
> -#define __ARM64_KVM_RAS_H__
> -
> -#include <linux/acpi.h>
> -#include <linux/errno.h>
> -#include <linux/types.h>
> -
> -#include <asm/acpi.h>
> -
> -/*
> - * Was this synchronous external abort a RAS notification?
> - * Returns '0' for errors handled by some RAS subsystem, or -ENOENT.
> - */
> -static inline int kvm_handle_guest_sea(void)
> -{
> - /* apei_claim_sea(NULL) expects to mask interrupts itself */
> - lockdep_assert_irqs_enabled();
> -
> - return apei_claim_sea(NULL);
> -}
> -
> -#endif /* __ARM64_KVM_RAS_H__ */
> diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> index 1c78864767c5..6934f4acdc45 100644
> --- a/arch/arm64/kvm/mmu.c
> +++ b/arch/arm64/kvm/mmu.c
> @@ -4,19 +4,20 @@
> * Author: Christoffer Dall <c.dall at virtualopensystems.com>
> */
>
> +#include <linux/acpi.h>
> #include <linux/mman.h>
> #include <linux/kvm_host.h>
> #include <linux/io.h>
> #include <linux/hugetlb.h>
> #include <linux/sched/signal.h>
> #include <trace/events/kvm.h>
> +#include <asm/acpi.h>
> #include <asm/pgalloc.h>
> #include <asm/cacheflush.h>
> #include <asm/kvm_arm.h>
> #include <asm/kvm_mmu.h>
> #include <asm/kvm_pgtable.h>
> #include <asm/kvm_pkvm.h>
> -#include <asm/kvm_ras.h>
> #include <asm/kvm_asm.h>
> #include <asm/kvm_emulate.h>
> #include <asm/virt.h>
> @@ -1811,6 +1812,20 @@ static void handle_access_fault(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa)
> read_unlock(&vcpu->kvm->mmu_lock);
> }
>
> +int kvm_handle_guest_sea(struct kvm_vcpu *vcpu)
> +{
> + /*
> + * Give APEI the opportunity to claim the abort before handling it
> + * within KVM. apei_claim_sea() expects to be called with IRQs
> + * enabled.
> + */
> + lockdep_assert_irqs_enabled();
> + if (apei_claim_sea(NULL) == 0)
> + return 1;
> +
> + return kvm_inject_serror(vcpu);
> +}
> +
> /**
> * kvm_handle_guest_abort - handles all 2nd stage aborts
> * @vcpu: the VCPU pointer
> @@ -1834,17 +1849,8 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu)
> gfn_t gfn;
> int ret, idx;
>
> - /* Synchronous External Abort? */
> - if (kvm_vcpu_abt_issea(vcpu)) {
> - /*
> - * For RAS the host kernel may handle this abort.
> - * There is no need to pass the error into the guest.
> - */
> - if (kvm_handle_guest_sea())
> - return kvm_inject_serror(vcpu);
> -
> - return 1;
> - }
> + if (kvm_vcpu_abt_issea(vcpu))
> + return kvm_handle_guest_sea(vcpu);
>
> esr = kvm_vcpu_get_esr(vcpu);
>
> diff --git a/arch/arm64/kvm/nested.c b/arch/arm64/kvm/nested.c
> index 096747a61bf6..38b0e3a9a6db 100644
> --- a/arch/arm64/kvm/nested.c
> +++ b/arch/arm64/kvm/nested.c
> @@ -1289,6 +1289,9 @@ int kvm_handle_vncr_abort(struct kvm_vcpu *vcpu)
>
> BUG_ON(!(esr & ESR_ELx_VNCR_SHIFT));
>
> + if (kvm_vcpu_abt_issea(vcpu))
> + return kvm_handle_guest_sea(vcpu);
> +
> if (esr_fsc_is_permission_fault(esr)) {
> inject_vncr_perm(vcpu);
> } else if (esr_fsc_is_translation_fault(esr)) {
> --
> 2.39.5
>
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