Re: [Xen-devel] [Qemu-devel] [PATCH] increase maxmem before calling xc_domain_populate_physmap

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 3 Dec 2014, Don Slutz wrote:
> On 12/03/14 07:20, Stefano Stabellini wrote:
> > On Wed, 3 Dec 2014, Wei Liu wrote:
> > > On Tue, Dec 02, 2014 at 03:23:29PM -0500, Don Slutz wrote:
> > > [...]
> > > > > > > >            hw_error("xc_domain_getinfo failed");
> > > > > > > >        }
> > > > > > > > -    if (xc_domain_setmaxmem(xen_xc, xen_domid, info.max_memkb +
> > > > > > > > -                            (nr_pfn * XC_PAGE_SIZE / 1024)) <
> > > > > > > > 0) {
> > > > > > > > +    max_pages = info.max_memkb * 1024 / XC_PAGE_SIZE;
> > > > > > > > +    free_pages = max_pages - info.nr_pages;
> > > > > > > > +    real_free = free_pages;
> > > > > > > > +    if (free_pages > VGA_HOLE_SIZE) {
> > > > > > > > +        free_pages -= VGA_HOLE_SIZE;
> > > > > > > > +    } else {
> > > > > > > > +        free_pages = 0;
> > > > > > > > +    }
> > > > > I don't think we need to subtract VGA_HOLE_SIZE.
> > > > If you do not use some slack (like 32 here), xen does report:
> > > > 
> > > > 
> > > > (d5) [2014-11-29 17:07:21] Loaded SeaBIOS
> > > > (d5) [2014-11-29 17:07:21] Creating MP tables ...
> > > > (d5) [2014-11-29 17:07:21] Loading ACPI ...
> > > > (XEN) [2014-11-29 17:07:21] page_alloc.c:1568:d5 Over-allocation for
> > > > domain
> > > > 5: 1057417 > 1057416
> > > > (XEN) [2014-11-29 17:07:21] memory.c:158:d5 Could not allocate order=0
> > > This message is a bit red herring.
> > > 
> > > It's hvmloader trying to populate ram for firmware data. The actual
> > > amount of extra pages needed depends on the firmware.
> > > 
> > > In any case it's safe to disallow hvmloader from doing so, it will just
> > > relocate some pages from ram (hence shrinking *mem_end).
> > That looks like a better solution
> > 
> 
> I went with a "leave some slack" so that the error message above is not
> output.
> 
> When a change to hvmloader is done so that the message does not appear during
> normal usage, the extra pages in QEMU can be dropped.

Although those messages look like fatal errors, they are not. It is
normal way for hvmloader to operate: firstly it tries to allocate extra
memory until it gets an error, then it continues with normal memory,
see:

void mem_hole_populate_ram(xen_pfn_t mfn, uint32_t nr_mfns)
{
    static int over_allocated;
    struct xen_add_to_physmap xatp;
    struct xen_memory_reservation xmr;

    for ( ; nr_mfns-- != 0; mfn++ )
    {
        /* Try to allocate a brand new page in the reserved area. */
        if ( !over_allocated )
        {
            xmr.domid = DOMID_SELF;
            xmr.mem_flags = 0;
            xmr.extent_order = 0;
            xmr.nr_extents = 1;
            set_xen_guest_handle(xmr.extent_start, &mfn);
            if ( hypercall_memory_op(XENMEM_populate_physmap, &xmr) == 1 )
                continue;
            over_allocated = 1;
        }

        /* Otherwise, relocate a page from the ordinary RAM map. */

I think there is really nothing to change there. Maybe we want to make
those warnings less scary.

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