[Xen-devel] Re: Early test for 32- vs 64-bit hvm domain?

[Andi Kleen <andi@xxxxxxxxxxxxxx>]

Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> writes:

> On 30/1/08 00:34, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:
>
>>> As long as e.g. Linux has not started (i.e. during bios and grub for
>>> instance), you can't know whether that will be a 64bit Linux
>>> or a 32bit
>>> Linux.
>> 
>> That's what I was afraid of...
>> 
>> Then what is the earliest point in the execution of an x86_64
>> domain/vcpu in *xen* where hvm_long_mode_enabled(v) will work
>> properly? E.g. in Linux/x86_64 is long mode set early in __start?
>> And is this done via a hypercall or an emulated instruction?
>
> As sson as you see LME bit get set in the EFER MSR then you know you have a
> 64-bit guest. Writes to EFER are emulated in long_mode_do_msr_write() in
> vmx.c and svm.c. Those are the places to patch in Xen 3.1 branch.

You could do it slightly earlier by intercepting bios int 0x15 eax=0xec00 ebx=2 
The kernel issues that to tell the BIOS it is 64bit.

That will only work if the boot loader does not skip the real mode code
(a few do) 

-Andi

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