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 ++++-
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,
gpa_t addr, unsigned int len)
{
struct vgic_cpu *vgic_cpu = &vcpu->arch.vgic_cpu;
+ u64 value = vgic_cpu->pendbaser;
- return extract_bytes(vgic_cpu->pendbaser, addr & 7, len);
+ value &= ~GICR_PENDBASER_PTZ;
+
+ return extract_bytes(value, addr & 7, len);
}
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.
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()).
This patch on its own fixes more than one bug!
Thanks,
M.
--
Jazz is not dead. It just smells funny...
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