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Re: [Xen-devel] [patch 1/2] HV: allow HVM virtual PICs to have their interrupt vector reprogrammed


On Thu, 2007-05-31 at 21:35 +0100, Stephen C. Tweedie wrote:

> If we are willing to extend that
> hard-coding into the hypervisor (but JUST the existing vmxassist-
> specific portion, not the general vPIC code), then the patch reduces to
> just a few lines, as below.

Reposted, this time complete with commit header and signed-off-by.

Fix boot loader hangs with syslinux's 32-bit vesamenu module.

Syslinux can load 32-bit UI code for menu handling.  But the core of
syslinux is still 16-bit.  When it jumps to this 32-bit code, it
installs a set of 32-bit interrupt trap handlers which just bounce the
interrupts back to 16-bit mode.

But this plays badly with vmxassist.  When running 16-bit boot loader
code, vmxassist installs its own trap handlers which bounce vPIC
interrupts back down to 16-bit mode.  The trap handlers live at int0x20
to 0x2f, so when the 16-bit code tries to reprogram the vPIC, vm86
rewrites the outb()s on the fly to set the irq_base vectors accordingly.

So when syslinux enters 32-bit mode, the vPIC has still been programmed
to point to vmxassist's bounce traps, even though vmxassist is no longer
active once the guest is running 32-bit code.  So the wrong interrupts
get delivered to the guest.

Fix this by restoring the rombios vPIC irq_base vectors when we leave
vmxassist mode, and returning the vmxassist traps when we reenter it.
These irq base values are hard-coded in this patch, but they are already
hard-coded in vmxassist so any boot code that relies on changing them
will already fail.

Signed-off-by: Stephen Tweedie <sct@xxxxxxxxxx>

--- xen/arch/x86/hvm/vmx/vmx.c.~2~      2007-05-03 17:49:31.000000000 +0100
+++ xen/arch/x86/hvm/vmx/vmx.c  2007-05-31 20:48:58.000000000 +0100
@@ -1914,6 +1914,20 @@ static int vmx_assist(struct vcpu *v, in
             if ( vmx_world_restore(v, &c) != 0 )
                 goto error;
             v->arch.hvm_vmx.vmxassist_enabled = 1;
+            /* 
+             * The 32-bit vmxassist vm86.c support code is hard-coded to
+             * expect vPIC interrupts to arrive at interrupt traps 0x20
+             * and 0x28.  It bounces these to 16-bit boot code offset
+             * from traps 0x08 and 0x70.  But when the guest transitions
+             * to true native 32-bit mode, vmxassist steps out of the
+             * way and no such bouncing occurs; so we need to rewrite
+             * the vPIC irq base to point direcetly to 0x08/0x70 (see
+             * code just below).  So on re-entering 16-bit mode, we need
+             * to reset the vPICs to go back to the 0x20/0x28 bounce
+             * traps.
+             */
+            v->domain->arch.hvm_domain.vpic[0].irq_base = 0x20;
+            v->domain->arch.hvm_domain.vpic[1].irq_base = 0x28;
             return 1;
@@ -1932,6 +1946,11 @@ static int vmx_assist(struct vcpu *v, in
             if ( vmx_world_restore(v, &c) != 0 )
                 goto error;
             v->arch.hvm_vmx.vmxassist_enabled = 0;
+            /* See comment above about vmxassist 16/32-bit vPIC
+             * behaviour.  The irq_base values are hard-coded into
+             * vmxassist vm86.c. */
+            v->domain->arch.hvm_domain.vpic[0].irq_base = 0x08;
+            v->domain->arch.hvm_domain.vpic[1].irq_base = 0x70;
             return 1;
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