linux-kernel - Re: [PATCH stable-5.12.y backport 1/2] KVM: arm64: Commit pending PC adjustemnts before returning to userspace

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Date:   Tue, 01 Jun 2021 12:44:28 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Zenghui Yu <yuzenghui@...wei.com>
Cc:     <stable@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <gregkh@...uxfoundation.org>, <sashal@...nel.org>,
        <alexandru.elisei@....com>, <wanghaibin.wang@...wei.com>
Subject: Re: [PATCH stable-5.12.y backport 1/2] KVM: arm64: Commit pending PC adjustemnts before returning to userspace

Hi Zenghui,

Thanks for having a go at the backport.

On Tue, 01 Jun 2021 12:12:37 +0100,
Zenghui Yu <yuzenghui@...wei.com> wrote:
> 
> From: Marc Zyngier <maz@...nel.org>
> 
> commit 26778aaa134a9aefdf5dbaad904054d7be9d656d upstream.
> 
> KVM currently updates PC (and the corresponding exception state)
> using a two phase approach: first by setting a set of flags,
> then by converting these flags into a state update when the vcpu
> is about to enter the guest.
> 
> However, this creates a disconnect with userspace if the vcpu thread
> returns there with any exception/PC flag set. In this case, the exposed
> context is wrong, as userspace doesn't have access to these flags
> (they aren't architectural). It also means that these flags are
> preserved across a reset, which isn't expected.
> 
> To solve this problem, force an explicit synchronisation of the
> exception state on vcpu exit to userspace. As an optimisation
> for nVHE systems, only perform this when there is something pending.
> 
> Reported-by: Zenghui Yu <yuzenghui@...wei.com>
> Reviewed-by: Alexandru Elisei <alexandru.elisei@....com>
> Reviewed-by: Zenghui Yu <yuzenghui@...wei.com>
> Tested-by: Zenghui Yu <yuzenghui@...wei.com>
> Signed-off-by: Marc Zyngier <maz@...nel.org>
> Cc: stable@...r.kernel.org # 5.11
> [yuz: stable-5.12.y backport: add __KVM_HOST_SMCCC_FUNC___kvm_adjust_pc
>  macro manually and keep it consistent with mainline]

I'd rather you allocated a new number here, irrespective of what
mainline has (rational below).

> Signed-off-by: Zenghui Yu <yuzenghui@...wei.com>
> ---
>  arch/arm64/include/asm/kvm_asm.h   |  1 +
>  arch/arm64/kvm/arm.c               | 11 +++++++++++
>  arch/arm64/kvm/hyp/exception.c     |  4 ++--
>  arch/arm64/kvm/hyp/nvhe/hyp-main.c |  8 ++++++++
>  4 files changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> index a8578d650bb6..d7f769bb6c9c 100644
> --- a/arch/arm64/include/asm/kvm_asm.h
> +++ b/arch/arm64/include/asm/kvm_asm.h
> @@ -57,6 +57,7 @@
>  #define __KVM_HOST_SMCCC_FUNC___kvm_get_mdcr_el2		12
>  #define __KVM_HOST_SMCCC_FUNC___vgic_v3_save_aprs		13
>  #define __KVM_HOST_SMCCC_FUNC___vgic_v3_restore_aprs		14
> +#define __KVM_HOST_SMCCC_FUNC___kvm_adjust_pc			21

This is going to generate a larger than necessary host_hcall array in
hyp/nvhe/hyp-main.c, which we're trying to keep tightly packed for
obvious reasons.

With this nit fixed:

Reviewed-by: Marc Zyngier <maz@...nel.org>

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.