[Qemu-devel] [PATCH] Save 64-bit of the IA-32e capable sysenter MSRs

[Qemu-devel] [PATCH] Save 64-bit of the IA-32e capable sysenter MSRs

[Alexander Graf]

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Hi,

When transitioning from KVM to the qemu userspace, we try to get and 
push a whole bunch of MSR values, including the SYSENTER ones. While 
this is basically a good idea, qemu doesn't know anything about SYSENTER 
on x86_64, which is an additional feature only available on Intel CPUs 
and thus defines SYSENTER_EIP and SYSENTER_ESP as 32-bit values.

Because we're saving/restoring the SYSENTER values on transitions to 
userspace and the values in env->cpu only hold 32 bits, they get 
truncated to 32-bit. The easiest way around this is to widen the fields 
to 64 bits. This should not disturb the current functionality, but 
allows us to run 32-bit code on x86_64 using VMX.

As this is KVM specific for now, I'll only CC the qemu mailinglist, but 
wouldn't encourage qemu to take the patch for now, as it's not necessary 
when emulating an AMD CPU.

Signed-off-by: Alexander Graf <agraf@suse.de>



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diff --git a/qemu/target-i386/cpu.h b/qemu/target-i386/cpu.h
index 7e95900..61c39d4 100644
--- a/qemu/target-i386/cpu.h
+++ b/qemu/target-i386/cpu.h
@@ -542,8 +542,8 @@ typedef struct CPUX86State {
 
     /* sysenter registers */
     uint32_t sysenter_cs;
-    uint32_t sysenter_esp;
-    uint32_t sysenter_eip;
+    uint64_t sysenter_esp;
+    uint64_t sysenter_eip;
     uint64_t efer;
     uint64_t star;
 

[Qemu-devel] Re: [PATCH] Save 64-bit of the IA-32e capable sysenter MSRs

[Avi Kivity]

Alexander Graf wrote:
> Hi,
>
> When transitioning from KVM to the qemu userspace, we try to get and 
> push a whole bunch of MSR values, including the SYSENTER ones. While 
> this is basically a good idea, qemu doesn't know anything about 
> SYSENTER on x86_64, which is an additional feature only available on 
> Intel CPUs and thus defines SYSENTER_EIP and SYSENTER_ESP as 32-bit 
> values.
>
> Because we're saving/restoring the SYSENTER values on transitions to 
> userspace and the values in env->cpu only hold 32 bits, they get 
> truncated to 32-bit. The easiest way around this is to widen the 
> fields to 64 bits. This should not disturb the current functionality, 
> but allows us to run 32-bit code on x86_64 using VMX.
>
> As this is KVM specific for now, I'll only CC the qemu mailinglist, 
> but wouldn't encourage qemu to take the patch for now, as it's not 
> necessary when emulating an AMD CPU.
>

Applied, thanks.  I imagine save/restore support needs updating as 
well?  That should go into qemuy btw since maintaining the save/restore 
format across forks isn't possible.

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

[Qemu-devel] Re: [PATCH] Save 64-bit of the IA-32e capable sysenter MSRs

[Alexander Graf]

On Jul 31, 2008, at 12:07 PM, Avi Kivity wrote:

> Alexander Graf wrote:
>> Hi,
>>
>> When transitioning from KVM to the qemu userspace, we try to get  
>> and push a whole bunch of MSR values, including the SYSENTER ones.  
>> While this is basically a good idea, qemu doesn't know anything  
>> about SYSENTER on x86_64, which is an additional feature only  
>> available on Intel CPUs and thus defines SYSENTER_EIP and  
>> SYSENTER_ESP as 32-bit values.
>>
>> Because we're saving/restoring the SYSENTER values on transitions  
>> to userspace and the values in env->cpu only hold 32 bits, they get  
>> truncated to 32-bit. The easiest way around this is to widen the  
>> fields to 64 bits. This should not disturb the current  
>> functionality, but allows us to run 32-bit code on x86_64 using VMX.
>>
>> As this is KVM specific for now, I'll only CC the qemu mailinglist,  
>> but wouldn't encourage qemu to take the patch for now, as it's not  
>> necessary when emulating an AMD CPU.
>>
>
> Applied, thanks.  I imagine save/restore support needs updating as  
> well?

Yes, it does. Thanks for the reminder.

>  That should go into qemuy btw since maintaining the save/restore  
> format across forks isn't possible.

I'll just write up a patch that implements IA-32e sysenter in qemu  
after vacation. That should fix the KVM issues too. For now it's good  
we have a workaround that works in "most cases" at least.

Alex

[Qemu-devel] Re: [PATCH] Save 64-bit of the IA-32e capable sysenter MSRs

[Avi Kivity]

Alexander Graf wrote:
>
> For now it's good we have a workaround that works in "most cases" at 
> least.

That's what the DANGEROUS label is for.


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