Re: [kvm-devel] [PATCH] Refactor hypercall infrastructure (v2)

Avi Kivity
2007-12-03T08:57:13Z

Amit Shah wrote:
> * Anthony Liguori wrote:
>
>> Amit Shah wrote:
>>
>>> * Anthony Liguori wrote:
>>>
>>>
>>>
>>>> This patch refactors the current hypercall infrastructure to better
>>>> support live migration and SMP. It eliminates the hypercall page by
>>>> trapping the UD exception that would occur if you used the wrong
>>>> hypercall instruction for the underlying architecture and replacing it
>>>> with the right one lazily.
>>>>
>>> This doesn't work right for SVM. It keeps looping indefinitely; on a
>>> kvm_stat run, I get about 230,000 light vm exits per second, with the
>>> hypercall never returning to the guest.
>>>
>>> ...
>>>
>>>
>> What are you using to issue the hypercall?
>>
>
> + r = kvm_hypercall1(KVM_PV_PCI_DEVICE, page_gfn);
>
> Setup is done by:
>
> + if (!kvm_para_available()) {
> + printk(KERN_ERR "KVM paravirt support not available\n");
> + r = -ENODEV;
> + goto out_dereg;
> + }
>

There was a bug where instructions with a modrm byte specifying a
register would try to access memory. In the memory was not mapped,
emulation would fail. vmcall is one such instruction. This was fixed by

commit f83562246921d6a8a7de8b76853a6835ace3699d
Author: Aurelien Jarno <aurelien@aurel32.net>
Date: Wed Oct 17 19:30:41 2007 +0200

KVM: x86 emulator: fix access registers for instructions with ModR/M
byte and Mod = 3

The patch belows changes the access type to register from memory for
instructions that are declared as SrcMem or DstMem, but have a
ModR/M byte with Mod = 3.

It fixes (at least) the lmsw and smsw instructions on an AMD64 CPU,
which are needed for FreeBSD.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Avi Kivity <avi@qumranet.com>

diff --git a/drivers/kvm/x86_emulate.c b/drivers/kvm/x86_emulate.c
index 7c95ae5..8c50496 100644
--- a/drivers/kvm/x86_emulate.c
+++ b/drivers/kvm/x86_emulate.c
@@ -835,6 +835,14 @@ modrm_done:
if (c->twobyte && c->b == 0x01
&& c->modrm_reg == 7)
break;
+ /*
+ * For instructions with a ModR/M byte, switch to register
+ * access if Mod = 3.
+ */
+ if ((c->d & ModRM) && c->modrm_mod == 3) {
+ c->src.type = OP_REG;
+ break;
+ }
srcmem_common:
c->src.type = OP_MEM;
break;
@@ -897,7 +905,14 @@ srcmem_common:
}
break;
case DstMem:
- c->dst.type = OP_MEM;
+ /*
+ * For instructions with a ModR/M byte, switch to register
+ * access if Mod = 3.
+ */
+ if ((c->d & ModRM) && c->modrm_mod == 3)
+ c->dst.type = OP_REG;
+ else
+ c->dst.type = OP_MEM;
break;
}

--
error compiling committee.c: too many arguments to function