reading IOPL

reading IOPL

[Jan Beulich]

Both Linux and Xen offer only ways to set the IOPL. While on native Linux
this is not a problem since user mode code can read the IOPL by inspecting
EFLAGS, on Xen this would always yield zero.

Now X folks appear to be using save-modify-restore cycles in some newer
code, and this obviously breaks under Xen, as they would restore IOPL 0
even when IOPL 3 was in effect before.

Since the only way to read the IOPL is xc_vcpu_getcontext() (i.e.
XEN_DOMCTL_getvcpucontext), I wonder whether we shouldn't add
e.g. a "get" counterpart to PHYSDEVOP_set_iopl.

Or am I overlooking some other access mechanism?

Jan

Re: reading IOPL

[Konrad Rzeszutek Wilk]

On Thu, May 03, 2012 at 05:07:04PM +0100, Jan Beulich wrote:
> Both Linux and Xen offer only ways to set the IOPL. While on native Linux
> this is not a problem since user mode code can read the IOPL by inspecting
> EFLAGS, on Xen this would always yield zero.
> 
> Now X folks appear to be using save-modify-restore cycles in some newer
> code, and this obviously breaks under Xen, as they would restore IOPL 0

This is in the KMS drivers or in the user-land Xorg?

> even when IOPL 3 was in effect before.
> 
> Since the only way to read the IOPL is xc_vcpu_getcontext() (i.e.
> XEN_DOMCTL_getvcpucontext), I wonder whether we shouldn't add
> e.g. a "get" counterpart to PHYSDEVOP_set_iopl.
> 
> Or am I overlooking some other access mechanism?
> 
> Jan
> 
> 
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xen.org
> http://lists.xen.org/xen-devel

Re: reading IOPL

[Jan Beulich]

>>> On 03.05.12 at 19:22, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
> On Thu, May 03, 2012 at 05:07:04PM +0100, Jan Beulich wrote:
>> Both Linux and Xen offer only ways to set the IOPL. While on native Linux
>> this is not a problem since user mode code can read the IOPL by inspecting
>> EFLAGS, on Xen this would always yield zero.
>> 
>> Now X folks appear to be using save-modify-restore cycles in some newer
>> code, and this obviously breaks under Xen, as they would restore IOPL 0
> 
> This is in the KMS drivers or in the user-land Xorg?

Userland code (as explained above).

Jan

>> even when IOPL 3 was in effect before.
>> 
>> Since the only way to read the IOPL is xc_vcpu_getcontext() (i.e.
>> XEN_DOMCTL_getvcpucontext), I wonder whether we shouldn't add
>> e.g. a "get" counterpart to PHYSDEVOP_set_iopl.
>> 
>> Or am I overlooking some other access mechanism?
>> 
>> Jan
>> 
>> 
>> 
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xen.org 
>> http://lists.xen.org/xen-devel