Re: [Qemu-devel] vhost-user: broken mem regions

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jun 26, 2014 at 07:44:24AM +0000, Damjan Marion (damarion) wrote:
> 
> On 26 Jun 2014, at 09:13, Michael S. Tsirkin <mst@redhat.com> wrote:
> 
> > On Wed, Jun 25, 2014 at 09:52:09PM +0000, Damjan Marion (damarion) wrote:
> >> 
> >> On 25 Jun 2014, at 18:44, Paolo Bonzini <pbonzini@redhat.com> wrote:
> >> 
> >>>> nregions: 4
> >>>> region:
> >>>> 	gpa = 0x100000000
> >>>> 	size = 3221225472
> >>>> 	ua = 0x2aab6ac00000
> >>> 
> >>> High memory, above 3 gigabytes.
> >>> 
> >>>> region:
> >>>> 	gpa = 0xFFFC0000
> >>>> 	size = 262144
> >>>> 	ua = 0x7fc13d200000
> >>> 
> >>> This is the BIOS.  There shouldn't be any FD for this one, it
> >>> is not allocated in hugetlbfs.
> >>> 
> >>>> region:
> >>>> 	gpa = 0x0
> >>>> 	size = 655360
> >>>> 	ua = 0x2aaaaac00000
> >>>> region:
> >>>> 	gpa = 0xC0000
> >>>> 	size = 3220439040
> >>>> 	ua = 0x2aaaaacc0000
> >>> 
> >>> Together, it's the first 3 GB of memory.
> >>> 
> >>> I understand now what you mean.  Yeah, the format should be changed
> >>> to include the offset (why does vhost-user need the ua at all?
> >>> perhaps the offset can replace the ua).
> >>> 
> >>>> BTW Any idea what to do with region 2, it doesn’t look like the one belonging
> >>>> to the same place, but qemu_get_ram_fd() returns same FD for it.
> >>> 
> >>> This must be a bug.  I would have expected qemu_get_ram_fd to return -1
> >>> here, so no descriptor should be passed to vhost-user.
> >> 
> >> Problem is inside qemu_get_ram_block():
> >> 
> >> 
> >>    if (block && addr - block->offset < block->length) {
> >>        goto found;
> >>    }
> >> 
> >> this is true if we have > 4G of RAM allocated.
> > 
> > Hmm I don't get it. Why is it always true for >4G RAM?
> 
> This check assumes that guest memory is mapped to RAM block 
> without gaps. as BIOS is mapped to guest address space
> near 4G this check will return true if block size is bigger than
> BIOS address.


But then offset is > addr so addr - offset is a huge value no?

> > 
> >> 
> >> Any advice how to fix this?
> >> 
> >>