[PATCH v15 1/5] arm/vpci: honor access size when returning an error

[Stewart Hildebrand <stewart.hildebrand@xxxxxxx>]

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>
---
v14->v15:
* re-order so this patch comes before ("xen/arm: translate virtual PCI
  bus topology for guests")
* s/access_mask/invalid/
* add U suffix to 1
* s/uint8_t/unsigned int/
* s/uint64_t/register_t/
* although Julien gave an Acked-by on v14, I omitted it due to the
  changes made in v15

v9->10:
* New patch in v10.
---
 xen/arch/arm/vpci.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/xen/arch/arm/vpci.c b/xen/arch/arm/vpci.c
index 3bc4bb55082a..b63a356bb4a8 100644
--- a/xen/arch/arm/vpci.c
+++ b/xen/arch/arm/vpci.c
@@ -29,6 +29,8 @@ static int vpci_mmio_read(struct vcpu *v, mmio_info_t *info,
 {
     struct pci_host_bridge *bridge = p;
     pci_sbdf_t sbdf = vpci_sbdf_from_gpa(bridge, info->gpa);
+    const unsigned int access_size = (1U << info->dabt.size) * 8;
+    const register_t invalid = GENMASK_ULL(access_size - 1, 0);
     /* data is needed to prevent a pointer cast on 32bit */
     unsigned long data;
 
@@ -39,7 +41,7 @@ static int vpci_mmio_read(struct vcpu *v, mmio_info_t *info,
         return 1;
     }
 
-    *r = ~0ul;
+    *r = invalid;
 
     return 0;
 }
-- 
2.45.1