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Re: [PATCH v14 5/5] arm/vpci: honor access size when returning an error



Hi Stewart,

On 14/05/2024 15:33, Stewart Hildebrand wrote:
From: Volodymyr Babchuk <Volodymyr_Babchuk@xxxxxxxx>

Guest can try to read config space using different access sizes: 8,
16, 32, 64 bits. We need to take this into account when we are
returning an error back to MMIO handler, otherwise it is possible to
provide more data than requested: i.e. guest issues LDRB instruction
to read one byte, but we are writing 0xFFFFFFFFFFFFFFFF in the target
register.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
Signed-off-by: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>

With one remark below:

Acked-by: Julien Grall <jgrall@xxxxxxxxxx>

---
v9->10:
* New patch in v10.
---
  xen/arch/arm/vpci.c | 6 ++++--
  1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/xen/arch/arm/vpci.c b/xen/arch/arm/vpci.c
index 348ba0fbc860..aaf9d9120c3d 100644
--- a/xen/arch/arm/vpci.c
+++ b/xen/arch/arm/vpci.c
@@ -41,6 +41,8 @@ static int vpci_mmio_read(struct vcpu *v, mmio_info_t *info,
  {
      struct pci_host_bridge *bridge = p;
      pci_sbdf_t sbdf;
+    const uint8_t access_size = (1 << info->dabt.size) * 8;
+    const uint64_t access_mask = GENMASK_ULL(access_size - 1, 0);
      /* data is needed to prevent a pointer cast on 32bit */
      unsigned long data;
@@ -48,7 +50,7 @@ static int vpci_mmio_read(struct vcpu *v, mmio_info_t *info, if ( !vpci_sbdf_from_gpa(v->domain, bridge, info->gpa, &sbdf) )
      {
-        *r = ~0UL;
+        *r = access_mask;

The name 'access_mask' is a bit confusing. I would not expect such value for be returned to the guest. What about 'invalid'?

Also can you confirm whether patches #4 and #5 be committed without the rest of the series?

Cheers,

--
Julien Grall



 


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