Re: QEMU on x64

[Peter Maydell]

On Fri, 30 Jul 2021 at 19:05, Christopher Caulfield
<ctcaulfield@gmail.com> wrote:
> This is Christopher from the debugging experiences team at Microsoft focused 
> on kernel debugging. I am reaching out with a few questions about QEMU on x64.
>
> Is it possible for the QEMU-x86-64 GDB Server to send the full set of x64 
> system registers (whether they are included in a separated system xml file or 
> as part of the core registers xml file)?

Do you mean "is it possible for somebody to write code for
QEMU to make it do that", or "does QEMU do it today if you pass
it the right command line option" ? The answer to the former
is "yes", to the latter "no". (If you want the debugger to
be able to write to the system registers this might be a little
trickier, mostly in terms of "auditing the code to make sure this
can't confuse QEMU if you change some sysreg under its feet.".)

> e.g. System registers missing from i386-64bit.xml file

> DWORD64 IDTBase;
> DWORD64 IDTLimit;
> DWORD64 GDTBase;
> DWORD64 GDTLimit;
> DWORD SelLDT;
> SEG64_DESC_INFO SegLDT;
> DWORD SelTSS;
> SEG64_DESC_INFO SegTSS;
>
> How can I access x64 MSR registers by using the QEMU-x86-64 GDB server?
>
> #define MSR_EFER 0xc0000080 // extended function enable register

EFER is in the xml ("x64_efer") so should be already accessible.
For anything else you're going to need to write some code to
make it happen.

>is there any plan to support reading/writing to MSRs via QEMU-x86-64 GDB 
>server?

Not that I know of. We'd be happy to review patches if you want to
write them.

thanks
-- PMM