Re: Limitation in HVM physmap

[Wei Liu <wei.liu2@citrix.com>]

On Fri, Oct 18, 2013 at 04:28:24PM +0200, Tim Deegan wrote:
> Hi,
> 
> At 15:20 +0100 on 18 Oct (1382106012), Wei Liu wrote:
> > I currently run into the limitation of HVM's physmap: one MFN can only
> > be mapped into one guest physical frame. Why is it designed like that?
> 
> 1. For simplicity.  That code is hard wnough to work with already. :)
> 
> 2. It helps avoid worrying about inconsistent cache settings if the
>    HAP tables only have one entry for each MFN. 
> 
> 3. Xen maintains a single mapping from MFN back to PFN, and any code
>    that uses it would have to be able to deal with getting multiple
>    answers.  That's already been partly changed by the mem_sharing
>    code (which obviously _can_ have multiple P2M entries pointing to
>    the same MFN but is a bit complex as a result).
> 

I see.

> > The scenario is that: when QEMU boots with OVMF (UEFI firmware), OVMF
> > will first map the framebuffer to 0x80000000, resulting the framebuffer
> > MFNs added to corresponding slots in physmap. A few moments later when
> > Linux kernel loads, it tries to map framebuffer MFNs to 0xf00000000,
> > which fails because those MFNs have already been mapped in other
> > locations. Is there a way to fix this?
> 
> Qemu could remember where it put the framebuffer last time and unmap
> it.  AIUI that's how real hardware would behave if you changed the
> framebuffer base address -- the framebuffer wouldn't still be mapped at
> the old location as well.
> 

In fact, this is not the case in OVMF. EFIFB driver in Linux will always
use the EFIFB region (0x80000000) provided by OVMF. 

Unmapping the first region (0x80000000) 1) makes the guest not able to
refresh its framebuffer, 2) causes lots of traps in QEMU (because guest
is still accessing first region) which makes QEMU busy-looping all the
time.

I do have a workaround in QEMU but it's very fragile and ugly...

Wei.

> Cheers,
> 
> Tim.