From: Marc Zyngier <maz@kernel.org>
To: Will Deacon <will@kernel.org>
Cc: kernel-team@android.com, kvmarm@lists.cs.columbia.edu,
linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/7] KVM: arm64: Update comment when skipping guest MMIO access instruction
Date: Sun, 26 Jul 2020 12:08:28 +0100 [thread overview]
Message-ID: <87v9iawn2r.wl-maz@kernel.org> (raw)
In-Reply-To: <20200724143506.17772-2-will@kernel.org>
On Fri, 24 Jul 2020 15:35:00 +0100,
Will Deacon <will@kernel.org> wrote:
>
> If a 32-bit guest accesses MMIO using a 16-bit Thumb-2 instruction that
> is reported to the hypervisor without a valid syndrom (for example,
> because of the addressing mode), then we may hand off the fault to
> userspace. When resuming the guest, we unconditionally advance the PC
> by 4 bytes, since ESR_EL2.IL is always 1 for data aborts generated without
> a valid syndrome. This is a bit rubbish, but it's also difficult to see
> how we can fix it without potentially introducing regressions in userspace
> MMIO fault handling.
Not quite, see below.
>
> Update the comment when skipping a guest MMIO access instruction so that
> this corner case is at least written down.
>
> Cc: Marc Zyngier <maz@kernel.org>
> Cc: Quentin Perret <qperret@google.com>
> Signed-off-by: Will Deacon <will@kernel.org>
> ---
> arch/arm64/kvm/mmio.c | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/arch/arm64/kvm/mmio.c b/arch/arm64/kvm/mmio.c
> index 4e0366759726..b54ea5aa6c06 100644
> --- a/arch/arm64/kvm/mmio.c
> +++ b/arch/arm64/kvm/mmio.c
> @@ -113,6 +113,13 @@ int kvm_handle_mmio_return(struct kvm_vcpu *vcpu, struct kvm_run *run)
> /*
> * The MMIO instruction is emulated and should not be re-executed
> * in the guest.
> + *
> + * Note: If user space handled the emulation because the abort
> + * symdrome information was not valid (ISV set in the ESR), then
nits: syndrome, ISV *clear*.
> + * this will assume that the faulting instruction was 32-bit.
> + * If the faulting instruction was a 16-bit Thumb instruction,
> + * then userspace would need to rewind the PC by 2 bytes prior to
> + * resuming the vCPU (yuck!).
> */
> kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
>
That's not how I read it. On ESR_EL2.ISV being clear, and in the
absence of KVM_CAP_ARM_NISV_TO_USER being set, we return a -ENOSYS from
io_mem_abort(), exiting back to userspace *without* advertising a MMIO
access. The VMM is free to do whatever it can to handle it (i.e. not
much), but crucially we don't go via kvm_handle_mmio_return() on
resuming the vcpu (unless the VMM sets run->exit_reason to
KVM_EXIT_MMIO, but that's clearly its own decision).
Instead, the expectation is that userspace willing to handle an exit
resulting in ESR_EL2.ISV being clear would instead request a
KVM_EXIT_ARM_NISV exit type (by enabling KVM_CAP_ARM_NISV_TO_USER),
getting extra information in the process such as as the fault
IPA). KVM_EXIT_ARM_NISV clearly states in the documentation:
"Note that KVM does not skip the faulting instruction as it does for
KVM_EXIT_MMIO, but userspace has to emulate any change to the
processing state if it decides to decode and emulate the instruction."
Thanks,
M.
--
Without deviation from the norm, progress is not possible.
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next prev parent reply other threads:[~2020-07-26 11:08 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-24 14:34 [PATCH 0/7] KVM: arm64: Fixes to early stage-2 fault handling Will Deacon
2020-07-24 14:35 ` [PATCH 1/7] KVM: arm64: Update comment when skipping guest MMIO access instruction Will Deacon
2020-07-26 11:08 ` Marc Zyngier [this message]
2020-07-27 10:30 ` Will Deacon
2020-07-24 14:35 ` [PATCH 2/7] KVM: arm64: Rename kvm_vcpu_dabt_isextabt() Will Deacon
2020-07-26 11:15 ` Marc Zyngier
2020-07-27 10:30 ` Will Deacon
2020-07-24 14:35 ` [PATCH 3/7] KVM: arm64: Handle data and instruction external aborts the same way Will Deacon
2020-07-24 14:35 ` [PATCH 4/7] KVM: arm64: Remove useless local variable Will Deacon
2020-07-24 14:35 ` [PATCH 5/7] KVM: arm64: Move 'invalid syndrome' logic out of io_mem_abort() Will Deacon
2020-07-26 11:55 ` Marc Zyngier
2020-07-27 10:31 ` Will Deacon
2020-07-24 14:35 ` [PATCH 6/7] KVM: arm64: Handle stage-2 faults on stage-1 page-table walks earlier Will Deacon
2020-07-26 13:38 ` Marc Zyngier
2020-07-27 10:29 ` Will Deacon
2020-07-24 14:35 ` [PATCH 7/7] KVM: arm64: Separate write faults on read-only memslots from MMIO Will Deacon
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