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, 1 Jun 2021 21:47:43 +0800
From:   Zenghui Yu <yuzenghui@...wei.com>
To:     Marc Zyngier <maz@...nel.org>
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 Marc,

On 2021/6/1 19:44, Marc Zyngier wrote:
> 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.

It isn't obvious to me ;-). But this creates some invalid entries
(HVC handlers) in the host_hcall array, which is not good. I'll change
__KVM_HOST_SMCCC_FUNC___kvm_adjust_pc to 15. Thanks for your reminder.

> With this nit fixed:
> 
> Reviewed-by: Marc Zyngier <maz@...nel.org>

Thanks!

Zenghui