Re: [PATCH] fix for Failed VMEntry on VMX

[David Lively <dlively@virtualiron.com>]

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Okay - revised patch attached.

Thanks,
Dave

Keir Fraser wrote:

>
> On 2 May 2006, at 15:58, David Lively wrote:
>
>> I've been getting a "Failed VMEntry" when trying to boot a second
>> VMX guest (while the first one is still running, but is no longer in
>> real mode).  This patch fixes it.
>
>
> Please `or' the vmread/vmwrite error returns together into an error 
> variable and BUG_ON(error) just once at the bottom of the function. 
> Also extend the comment a little to explicitly explain that this is 
> working around a VMENTRY validation check.
>
> Apart from that it looks good.
>
>  -- Keir
>


[-- Attachment #2: failed-vmentry-fix.patch --]
[-- Type: text/x-patch, Size: 2663 bytes --]

Ensure segment bases are consistent with their
selectors for VMX guests in VM86 mode.

Signed-off-by: David Lively <dlively@virtualiron.com>

diff -r 880433ba7487 xen/arch/x86/hvm/vmx/vmx.c
--- a/xen/arch/x86/hvm/vmx/vmx.c	Mon May  1 17:08:02 2006 -0400
+++ b/xen/arch/x86/hvm/vmx/vmx.c	Tue May  2 12:43:47 2006 -0400
@@ -487,6 +487,44 @@ static void vmx_store_cpu_guest_regs(
         __vmptrld(virt_to_maddr(current->arch.hvm_vmx.vmcs));
 }
 
+/* The VMX spec (section 4.3.1.2, Checks on Guest Segment
+ * Registers) says that virtual-8086 mode guests' segment
+ * base-address fields in the VMCS must be equal to their
+ * corresponding segment selector field shifted right by
+ * four bits upon vmentry.
+ *
+ * This function (called only for VM86-mode guests) fixes
+ * the bases to be consistent with the selectors in regs
+ * if they're not already.  Without this, we can fail the
+ * vmentry check mentioned above.
+ */
+static void fixup_vm86_seg_bases(struct cpu_user_regs *regs)
+{
+    int err = 0;
+    unsigned long base;
+
+    err |= __vmread(GUEST_ES_BASE, &base);
+    if (regs->es << 4 != base)
+        err |= __vmwrite(GUEST_ES_BASE, regs->es << 4);
+    err |= __vmread(GUEST_CS_BASE, &base);
+    if (regs->cs << 4 != base)
+        err |= __vmwrite(GUEST_CS_BASE, regs->cs << 4);
+    err |= __vmread(GUEST_SS_BASE, &base);
+    if (regs->ss << 4 != base)
+        err |= __vmwrite(GUEST_SS_BASE, regs->ss << 4);
+    err |= __vmread(GUEST_DS_BASE, &base);
+    if (regs->ds << 4 != base)
+        err |= __vmwrite(GUEST_DS_BASE, regs->ds << 4);
+    err |= __vmread(GUEST_FS_BASE, &base);
+    if (regs->fs << 4 != base)
+        err |= __vmwrite(GUEST_FS_BASE, regs->fs << 4);
+    err |= __vmread(GUEST_GS_BASE, &base);
+    if (regs->gs << 4 != base)
+        err |= __vmwrite(GUEST_GS_BASE, regs->gs << 4);
+
+    BUG_ON(err);
+}
+
 void vmx_load_cpu_guest_regs(struct vcpu *v, struct cpu_user_regs *regs)
 {
     if ( v != current )
@@ -523,6 +561,8 @@ void vmx_load_cpu_guest_regs(struct vcpu
         __vm_set_bit(EXCEPTION_BITMAP, EXCEPTION_BITMAP_DB);
     else
         __vm_clear_bit(EXCEPTION_BITMAP, EXCEPTION_BITMAP_DB);
+    if (regs->rflags & EF_VM)
+        fixup_vm86_seg_bases(regs);
 
     __vmwrite(GUEST_CS_SELECTOR, regs->cs);
     __vmwrite(GUEST_RIP, regs->rip);
@@ -540,6 +580,8 @@ void vmx_load_cpu_guest_regs(struct vcpu
         __vm_set_bit(EXCEPTION_BITMAP, EXCEPTION_BITMAP_DB);
     else
         __vm_clear_bit(EXCEPTION_BITMAP, EXCEPTION_BITMAP_DB);
+    if (regs->eflags & EF_VM)
+        fixup_vm86_seg_bases(regs);
 
     __vmwrite(GUEST_CS_SELECTOR, regs->cs);
     __vmwrite(GUEST_RIP, regs->eip);

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