RE: [PATCH v13 2/9] arm/arm64: KVM: Advertise KVM UID to guests via SMCCC

[Jianyong Wu <Jianyong.Wu@arm.com>]

> -----Original Message-----
> From: Will Deacon <will@kernel.org>
> Sent: Monday, July 27, 2020 7:38 PM
> To: Jianyong Wu <Jianyong.Wu@arm.com>
> Cc: netdev@vger.kernel.org; yangbo.lu@nxp.com; john.stultz@linaro.org;
> tglx@linutronix.de; pbonzini@redhat.com; sean.j.christopherson@intel.com;
> maz@kernel.org; richardcochran@gmail.com; Mark Rutland
> <Mark.Rutland@arm.com>; Suzuki Poulose <Suzuki.Poulose@arm.com>;
> Steven Price <Steven.Price@arm.com>; linux-kernel@vger.kernel.org; linux-
> arm-kernel@lists.infradead.org; kvmarm@lists.cs.columbia.edu;
> kvm@vger.kernel.org; Steve Capper <Steve.Capper@arm.com>; Kaly Xin
> <Kaly.Xin@arm.com>; Justin He <Justin.He@arm.com>; Wei Chen
> <Wei.Chen@arm.com>; nd <nd@arm.com>
> Subject: Re: [PATCH v13 2/9] arm/arm64: KVM: Advertise KVM UID to guests
> via SMCCC
> 
> On Mon, Jul 27, 2020 at 03:45:37AM +0000, Jianyong Wu wrote:
> > > From: Will Deacon <will@kernel.org>
> > >
> > > We can advertise ourselves to guests as KVM and provide a basic
> > > features bitmap for discoverability of future hypervisor services.
> > >
> > > Cc: Marc Zyngier <maz@kernel.org>
> > > Signed-off-by: Will Deacon <will@kernel.org>
> > > Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>
> > > ---
> > >  arch/arm64/kvm/hypercalls.c | 29 +++++++++++++++++++----------
> > >  1 file changed, 19 insertions(+), 10 deletions(-)
> > >
> > > diff --git a/arch/arm64/kvm/hypercalls.c
> > > b/arch/arm64/kvm/hypercalls.c index 550dfa3e53cd..db6dce3d0e23
> > > 100644
> > > --- a/arch/arm64/kvm/hypercalls.c
> > > +++ b/arch/arm64/kvm/hypercalls.c
> > > @@ -12,13 +12,13 @@
> > >  int kvm_hvc_call_handler(struct kvm_vcpu *vcpu)  {
> > >  	u32 func_id = smccc_get_function(vcpu);
> > > -	long val = SMCCC_RET_NOT_SUPPORTED;
> > > +	u32 val[4] = {SMCCC_RET_NOT_SUPPORTED};
> >
> > There is a risk as this u32 value will return here and a u64 value
> > will be obtained in guest. For example, The val[0] is initialized as
> > -1 of 0xffffffff and the guest get 0xffffffff then it will be compared
> > with -1 of 0xffffffffffffffff Also this problem exists for the
> > transfer of address in u64 type. So the following assignment to "val"
> > should be split into two
> > u32 value and assign to val[0] and val[1] respectively.
> > WDYT?
> 
> Yes, I think you're right that this is a bug, but isn't the solution just to make
> that an array of 'long'?
> 
> 	long val [4];
> 
> That will sign-extend the negative error codes as required, while leaving the
> explicitly unsigned UID constants alone.

Ok, that's much better. I will fix it at next version.

By the way, I wonder when will you update this patch set. I see someone like me
adopt this patch set as code base and need rebase it every time, so expect your update.

Thanks
Jianyong 
> 
> Will
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