Re: [PATCH] KVM: arm64/sve: Fix vq_present() macro to yield a bool

Paolo Bonzini
2019-07-04T12:24:47Z

On 04/07/19 10:20, Marc Zyngier wrote:
> +KVM, Paolo and Radim,
>
> Guys, do you mind picking this single patch and sending it to Linus?
> That's the only fix left for 5.2. Alternatively, I can send you a pull
> request, but it feels overkill.

Sure, will do.

Paolo

> Either way, please let me know.
>
> Thanks,
>
> M.
>
> On Wed, 03 Jul 2019 18:42:50 +0100,
> Dave Martin <Dave.Martin@arm.com> wrote:
>>
>> From: Zhang Lei <zhang.lei@jp.fujitsu.com>
>>
>> The original implementation of vq_present() relied on aggressive
>> inlining in order for the compiler to know that the code is
>> correct, due to some const-casting issues. This was causing sparse
>> and clang to complain, while GCC compiled cleanly.
>>
>> Commit 0c529ff789bc addressed this problem, but since vq_present()
>> is no longer a function, there is now no implicit casting of the
>> returned value to the return type (bool).
>>
>> In set_sve_vls(), this uncast bit value is compared against a bool,
>> and so may spuriously compare as unequal when both are nonzero. As
>> a result, KVM may reject valid SVE vector length configurations as
>> invalid, and vice versa.
>>
>> Fix it by forcing the returned value to a bool.
>>
>> Signed-off-by: Zhang Lei <zhang.lei@jp.fujitsu.com>
>> Fixes: 0c529ff789bc ("KVM: arm64: Implement vq_present() as a macro")
>> Signed-off-by: Dave Martin <Dave.Martin@arm.com> [commit message rewrite]
>> Cc: Viresh Kumar <viresh.kumar@linaro.org>
>>
>> ---
>>
>> Posting this under Zhang Lei's authorship, due to the need to turn this
>> fix around quickly. The fix is as per the original suggestion [1].
>>
>> Originally observed with the QEMU KVM SVE support under review:
>> https://lists.gnu.org/archive/html/qemu-devel/2019-06/msg04945.html
>>
>> Bug reproduced and fix tested on the Arm Fast Model, with
>> http://linux-arm.org/git?p=kvmtool-dm.git;a=shortlog;h=refs/heads/sve/v3/head
>> (After rerunning util/update_headers.sh.)
>>
>> (the --sve-vls command line argument was removed in v4 of the
>> kvmtool patches).
>>
>> [1] http://lists.infradead.org/pipermail/linux-arm-kernel/2019-July/664633.html
>> ---
>> arch/arm64/kvm/guest.c | 2 +-
>> 1 file changed, 1 insertion(+), 1 deletion(-)
>>
>> diff --git a/arch/arm64/kvm/guest.c b/arch/arm64/kvm/guest.c
>> index c2afa79..dfd6264 100644
>> --- a/arch/arm64/kvm/guest.c
>> +++ b/arch/arm64/kvm/guest.c
>> @@ -208,7 +208,7 @@ static int set_core_reg(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>>
>> #define vq_word(vq) (((vq) - SVE_VQ_MIN) / 64)
>> #define vq_mask(vq) ((u64)1 << ((vq) - SVE_VQ_MIN) % 64)
>> -#define vq_present(vqs, vq) ((vqs)[vq_word(vq)] & vq_mask(vq))
>> +#define vq_present(vqs, vq) (!!((vqs)[vq_word(vq)] & vq_mask(vq)))
>>
>> static int get_sve_vls(struct kvm_vcpu *vcpu, const struct kvm_one_reg *reg)
>> {
>> --
>> 2.1.4
>>
>