[edk2-devel] Does anyone know the definition of EFI_SYSTEM_CONTEXT_X64.Gdtr[2] and EFI_SYSTEM_CONTEXT_X64.Idtr[2]

[Michael D Kinney]

Hi Andrew,

I see the following assembly that reads GDTR and IDTR from
CPU and saves them into the context structure:

;; UINT32  Gdtr[2], Idtr[2];
    sub     esp, 8
    sidt    [esp]
    mov     eax, [esp + 2]
    xchg    eax, [esp]
    and     eax, 0xFFFF
    mov     [esp+4], eax

    sub     esp, 8
    sgdt    [esp]
    mov     eax, [esp + 2]
    xchg    eax, [esp]
    and     eax, 0xFFFF
    mov     [esp+4], eax

This data structure is defined in UEFI Spec, but I do not
see a detailed description of these 2 arrays and the meaning
of each entry.  Seems like a gap in the spec description.

Mike

> -----Original Message-----
> From: Andrew Fish <afish at apple.com>
> Sent: Wednesday, June 3, 2020 5:57 PM
> To: edk2-devel-groups-io <devel at edk2.groups.io>; Kinney,
> Michael D <michael.d.kinney at intel.com>
> Subject: Re: [edk2-devel] Does anyone know the
> definition of EFI_SYSTEM_CONTEXT_X64.Gdtr[2] and
> EFI_SYSTEM_CONTEXT_X64.Idtr[2]
> 
> 
> 
> > On Jun 3, 2020, at 5:37 PM, Michael D Kinney
> <michael.d.kinney at intel.com> wrote:
> >
> > Hi Andrew,
> >
> > Are you referring to the order the fields are filled
> in by the C code
> > before the x86 instruction is executed?  I do not
> think that matters.
> >
> 
> Mike,
> 
> The instructions point to a blob of data that is not
> UINTN aligned. These register point to a limit then base
> address. If you execute the instruction you get 2 bytes
> of limit and 4 or 8 bytes of data. But Idtr[0] is base
> and Intro[1] is limit, so reversed from the instruction
> order. What defines index 0 vs. index 1 data contents?
> 
> Thanks,
> 
> Andrew Fish
> 
> > Thanks,
> >
> > Mike
> >
> >> -----Original Message-----
> >> From: devel at edk2.groups.io <devel at edk2.groups.io> On
> >> Behalf Of Andrew Fish via groups.io
> >> Sent: Wednesday, June 3, 2020 4:39 PM
> >> To: edk2-devel-groups-io <devel at edk2.groups.io>
> >> Subject: [edk2-devel] Does anyone know the definition
> of
> >> EFI_SYSTEM_CONTEXT_X64.Gdtr[2] and
> >> EFI_SYSTEM_CONTEXT_X64.Idtr[2]
> >>
> >> The x86 instructions do limit then base, but the code
> is
> >> storing base, then limit. Does anyone remember if
> this
> >> is properly defined some place?
> >>
> >> Thanks,
> >>
> >> Andrew Fish
> >>
> >
> >
> > 
> >


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