Hi Marc,

On 2020/3/23 16:25, Marc Zyngier wrote:

Hi Zenghui,

[...]

quoted

quoted

And actually, maybe we can handle that pretty cheaply. If userspace
tries to restore GICD_TYPER2 to a value that isn't what KVM can
offer, we just return an error. Exactly like we do for GICD_IIDR.
Hence the following patch:

diff --git a/virt/kvm/arm/vgic/vgic-mmio-v3.c 

b/virt/kvm/arm/vgic/vgic-mmio-v3.c
index 28b639fd1abc..e72dcc454247 100644

--- a/virt/kvm/arm/vgic/vgic-mmio-v3.c
+++ b/virt/kvm/arm/vgic/vgic-mmio-v3.c

@@ -156,6 +156,7 @@ static int vgic_mmio_uaccess_write_v3_misc(struct 

kvm_vcpu *vcpu,
      struct vgic_dist *dist = &vcpu->kvm->arch.vgic;

      switch (addr & 0x0c) {
+    case GICD_TYPER2:
      case GICD_IIDR:
          if (val != vgic_mmio_read_v3_misc(vcpu, addr, len))
              return -EINVAL;

Being a RO register, writing something that isn't compatible with the
possible behaviour of the hypervisor will just return an error.

This is really a nice point to address my concern! I'm happy to see
this in v6 now.

quoted

What do you think?

I agreed with you, with a bit nervous though. Some old guests (which
have no knowledge about GICv4.1 vSGIs and don't care about nASSGIcap
at all) will also fail to migrate from A to B, just because now we
present two different (unused) GICD_TYPER2 registers to them.

Is it a little unfair to them :-) ?

I never pretended to be fair! ;-)

I'm happy to prevent migrating from a v4.1 system (A) to a v4.0
system (B). As soon as the guest has run, it isn't safe to do so
(it may have read TYPER2, and now knows about vSGIs). We *could*
track this and find ways to migrate this state as well, but it
feels fragile.

Migrating from B to A is more appealing. It should be possible to
do so without much difficulty (just check that the nASSGIcap bit
is either 0 or equal to KVM's view of the capability).

But overall I seriously doubt we can easily migrate guests across
very different HW. We've been talking about this for years, and
we still don't have a good solution for it given the diversity
of the ecosystem... :-/

Fair enough. Thanks for your detailed explanation!


Zenghui


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