Re: [Xen-devel] Getting the XSAVE size from userspace

[Andrei LUTAS <vlutas@xxxxxxxxxxxxxxx>]

On 11/5/2015 12:51 PM, Jan Beulich wrote:
> > > > On 05.11.15 at 11:49, <andrew.cooper3@xxxxxxxxxx> wrote:
> > On 05/11/15 10:42, Jan Beulich wrote:
> > > > > > On 05.11.15 at 10:52, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> > > > I need to get the XSAVE size from userspace. The easiest way seems to be
> > > > to use the XEN_DOMCTL_getvcpuextstate hypercall, but that hypercall is
> > > > not public / there's no xenctrl.h wrapper for it.
> > > Before going into any detail of the rest of your mail - any reason you
> > > can't just consult CPUID output?
> > It depends on precisely what you want.
> >
> > CPUID.0xD[0].ecx gives you the maximum xsave area on this processor
> > CPUID.0xD[0].ebx gives you the current size for the value in xcr0, but
> > that is not very useful from userspace.
> Why would the maximum size not be sufficient for most (all?) user
> mode purposes?
>
> Jan
>
>
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Hello,

The use-case is the following: whenever an EPT violation is triggered 
inside a monitored VM, the introspection logic needs to know how many 
bytes were accessed (read/written). This is done by inspecting the 
faulting instruction and directly inferring the size, which is not 
straight-forward for XSAVE/XRSTOR family. Using the maximum possible 
size is wrong, as in any given moment the OS may or may not desire to 
XSAVE/XRSTOR the entire state (and thinking that the instruction tries 
to access more than it actually does may yield undesired effects). 
Therefore, the size needed for the currently enabled features of the 
monitored guest is required instead. Normally, it could be done by 
running CPUID with eax = 0xD and ecx = i, where i >= 2 and XCR0[i] is 1 
(XCR0 belongs to the monitored guest), but I am unsure if using CPUID 
this way would be safe/desired: will Xen expose the same CPUID features, 
for XSAVE related functionality, on all VMs? (using XCPUID with eax = 
0xD and ecx = 0 would give us the needed size for the SVA, and like I 
said, using the maximum size would not be safe, even if it's the same 
across all VMs on a given host). Also, I'm unsure how this would get 
along with migration...
Thanks,
Andrei.

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