[xen staging] arm/vpci: honor access size when returning an error

[patchbot@xxxxxxx]

commit 7db7bd0f319fe7f19a6904e75c53adf891fa2dac
Author:     Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>
AuthorDate: Thu May 23 10:17:30 2024 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Thu May 23 10:17:30 2024 +0200

    arm/vpci: honor access size when returning an error
    
    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>
    Acked-by: Julien Grall <jgrall@xxxxxxxxxx>
---
 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 3bc4bb5508..b63a356bb4 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;
 }
--
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