[PATCH] x86/hvm: Rationalise CS limit handling in arch_set_info_hvm_guest()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

Ever since it's introduction in commit 192df6f9122d ("x86: allow HVM guests to
use hypercalls to bring up vCPUs"), %cs.g/limit has been handled differently
to all other segments.

The hypercall takes full 32bit, and hvm_set_segment_register() fixes up all
segments .g to match the limit being 2^20 or more.  Therefore, treating %cs
only as having architectural (20-bit) limit field is weird and unexpected.

Remove the custom handling for %cs.  This is a guest ABI change, but all
callers are expected to be setting up flat segmentation, at which point limit
will be ~0U and there will be no change in practice whether .g is set or not.

Reported-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Slightly RFC as this is an ABI change, but I don't anticipate any breakge from
this change.
---
 xen/arch/x86/hvm/domain.c | 8 ++------
 1 file changed, 2 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/hvm/domain.c b/xen/arch/x86/hvm/domain.c
index 048f29ae4911..4e9aaca39fe6 100644
--- a/xen/arch/x86/hvm/domain.c
+++ b/xen/arch/x86/hvm/domain.c
@@ -120,7 +120,6 @@ int arch_set_info_hvm_guest(struct vcpu *v, const struct 
vcpu_hvm_context *ctx)
     case VCPU_HVM_MODE_32B:
     {
         const struct vcpu_hvm_x86_32 *regs = &ctx->cpu_regs.x86_32;
-        uint32_t limit;
 
         if ( ctx->cpu_regs.x86_32.pad1 != 0 ||
              ctx->cpu_regs.x86_32.pad2[0] != 0 ||
@@ -147,13 +146,10 @@ int arch_set_info_hvm_guest(struct vcpu *v, const struct 
vcpu_hvm_context *ctx)
             return rc;
 
         /* Basic sanity checks. */
-        limit = cs.limit;
-        if ( cs.g )
-            limit = (limit << 12) | 0xfff;
-        if ( regs->eip > limit )
+        if ( regs->eip > cs.limit )
         {
             gprintk(XENLOG_ERR, "EIP (%#08x) outside CS limit (%#08x)\n",
-                    regs->eip, limit);
+                    regs->eip, cs.limit);
             return -EINVAL;
         }
 

base-commit: 3999ff0d307a9a901ad1b5ad56e0dde657fec558
-- 
2.39.5