Re: [PATCH] enable gfxboot on VMX

[Avi Kivity <avi@qumranet.com>]

Alexander Graf wrote:
> Hi,
>
> this issue has already been talked about previously. Gfxboot on VMX is
> broken, because it reads SS after switching from real to protected mode,
> where SS contains an invalid value, which VMX does not allow.
> As far as I know, gfxboot is the only application that suffers from this
> issue.
> The current "fix" is to make gfxboot use a previously stored SS value,
> which works fine for new releases. Already shipped versions of the
> software can not be changed though, so there needs to be another way to
> make kvm work with older versions of gfxboot.
>
> As everything except gfxboot works, we can simply change gfxboot in
> runtime to use a different value. Unfortunately the mov instruction,
> used to read the SS register is only 2 bytes long, so there is no way to
> binary patch the mov to something that would contain an address. So the
> only way I could think of was an invalid instruction. The UD exception
> is intercepted in KVM and is already emulated for VMCALLs. This can be
> extended to an opcode, that is officially unused (0f 0c) and have the
> emulator do a mov realmode_ss, %eax.
>
> This patch implements exactly this idea and fixes openSUSE < 11.0 and
> Ubuntu CD booting on VMX for me. Comments are, as always, welcome.
>   

While enabling gfxboot over vmx is very desirable, I'd like to avoid 
guest-specific hacks.  IMO the correct fix is to set a "non_vt_friendly" 
flag when switching from real mode to protected mode, then continue 
emulation, re-computing the flag after every instruction.  After a few 
instruction, the condition disappears and we can enter guest mode again.

The same approach works for big real mode.

The downside is that we have to implement more instructions in the 
emulator for this, but these instructions will be generally useful, not 
just for gfxboot.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.


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