Re: [Xen-devel] [PATCH v2 1/2] xc_cpuid_x86.c: Simplify masking conditions and remove redundant work

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.09.14 at 14:29, <andrew.cooper3@xxxxxxxxxx> wrote:
> On Intel, syscall is strictly only available in long mode, being an AMD
> instruction mandated in the 64bit spec.
> 
> is_64bit is disappearing as Xen is unconditionally 64bit these days, but
> preventing the guest using PAE will preclude it being able to enter long
> mode.
> 
> I would agree that it is not necessarily obvious, and based on this
> consideration, I think it would be better to keep the variable
> "is_64bit" as it is more informative than "is_pae" in the contexts used.

Or drop the conditional, as suggested before. After all I don't think
Intel CPUs advertise the bit in CPUID only when in long mode. Plus
even if they did, what we do (did) here still wouldn't match their
behavior (since is_64bit really is a hypervisor property, not a guest
one). I.e. at the tools level we should leave the flag as is. And if
indeed CPUID output changes when entering long mode (didn't
check the spec), this could only be mimicked in hypervisor code
anyway.

Jan


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