Re: [PATCH] KVM: arm/arm64: vgic: Handle GICR_PENDBASER.PTZ filed as RAZ

[Auger Eric]

Hi Zenghui,

On 12/24/19 3:52 AM, Zenghui Yu wrote:
> Hi Marc, Eric,
> 
> On 2019/12/23 22:07, Marc Zyngier wrote:
>> Hi Zenghui,
>>
>> On 2019-12-23 13:43, Zenghui Yu wrote:
>>> On 2019/12/20 19:18, Zenghui Yu wrote:
>>>> Although guest will hardly read and use the PTZ (Pending Table Zero)
>>>> bit in GICR_PENDBASER, let us emulate the architecture strictly.
>>>> As per IHI 0069E 9.11.30, PTZ field is WO, and reads as 0.
>>>> Signed-off-by: Zenghui Yu <yuzenghui@xxxxxxxxxx>
>>>> ---
>>>> Noticed when checking all fields of GICR_PENDBASER register.
>>>> But _not_ sure whether it's worth a fix, as Linux never sets
>>>> the PTZ bit before enabling LPI (set GICR_CTLR_ENABLE_LPIS).
>>>> And I wonder under which scenarios can this bit be written as 1.
>>>> It seems difficult for software to determine whether the pending
>>>> table contains all zeros when writing this bit.
>>>> virt/kvm/arm/vgic/vgic-mmio-v3.c | 5 ++++-
>>>> Â 1 file changed, 4 insertions(+), 1 deletion(-)
>>>> diff --git a/virt/kvm/arm/vgic/vgic-mmio-v3.c
>>>> b/virt/kvm/arm/vgic/vgic-mmio-v3.c
>>>> index 7dfd15dbb308..ebc218840fc2 100644
>>>> --- a/virt/kvm/arm/vgic/vgic-mmio-v3.c
>>>> +++ b/virt/kvm/arm/vgic/vgic-mmio-v3.c
>>>> @@ -414,8 +414,11 @@ static unsigned long
>>>> vgic_mmio_read_pendbase(struct kvm_vcpu *vcpu,
>>>> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ gpa_t addr, unsigned int len)
>>>> Â {
>>>> ÂÂÂÂÂ struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
>>>> +ÂÂÂ u64 value = vgic_cpu->pendbaser;
>>>> Â -ÂÂÂ return extract_bytes(vgic_cpu->pendbaser, addr & 7, len);
>>>> +ÂÂÂ value &= ~GICR_PENDBASER_PTZ;
>>>> +
>>>> +ÂÂÂ return extract_bytes(value, addr & 7, len);
>>>> Â }
>>>> Â static void vgic_mmio_write_pendbase(struct kvm_vcpu *vcpu,
>>>>
>>>
>>> I noticed there is no userspace access callbacks for GICR_PENDBASER,
>>> so this patch will make the PTZ field also 'Read As Zero' by userspace.
>>> Should we consider adding a uaccess_read callback for GICR_PENDBASER
>>> which just returns the unchanged vgic_cpu->pendbaser to userspace?
>>> (Though this is really not a big deal. We now always emulate the PTZ
>>> field to guest as RAZ. And 'vgic_cpu->pendbaser & GICR_PENDBASER_PTZ'
>>> only indicates whether KVM will optimize the LPI enabling process,
>>> where Read As Zero indicates never optimize..)
>>
>> I don't think adding a userspace accessor would help much. All this
>> bit tells userspace is that the guest has programmed a zero filled
>> table. On restore, we'd avoid a rescan of the table if there was
>> no LPI mapped.
> 
> Yes, I agree.
> 
>> And thinking of it, this fixes a bug for non-Linux guests: If you write
>> PTZ=1, we never clear it. Which means that if userspace saves and
>> restores
>> PENDBASER with PTZ set, we'll never restore the pending bits, which is
>> pretty bad (see vgic_enable_lpis()).
> 
> But I'm afraid I can't follow this point. After reading the code (with
> Qemu) a bit further, the Redistributors are restored before the ITS.

This is also part of the kernel documentation:
Documentation/virt/kvm/devices/arm-vgic-its.txt (ITS restore sequence)
 So
> there should be _no_ LPI has been mapped when we're restoring GICR_CTLR
> and enabling LPI, which says we will not scan the whole pending table
> and restore pending by vgic_enable_lpis()/its_sync_lpi_pending_table(),
> regardless of what the PTZ is.
> 
> Instead, vgic_its_restore_ite()/vgic_v3_lpi_sync_pending_status() is
> where we actually read the guest RAM and restore the LPI pending state.
yes the pending state is restored from
vgic_its_restore_ite/vgic_add_lpi/vgic_v3_lpi_sync_pending_status and
this path ignores the PTZ.

Thanks

Eric
> Which means we will still do the right thing even for non-Linux guests.
> Not sure if I've got things correctly here.
> 
> In the end, let's keep the patch as it is.
> 
>>
>> This patch on its own fixes more than one bug!
>>
> 
> If so, just by luck ;-)
> 
> 
> Thanks,
> Zenghui
> 
> 
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