Re: [Xen-devel] [PATCH for-4.12 4/8] x86/shadow: alloc enough pages so initialization doesn't fail

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Feb 05, 2019 at 04:21:52AM -0700, Jan Beulich wrote:
> >>> On 30.01.19 at 11:36, <roger.pau@xxxxxxxxxx> wrote:
> > Current code in shadow_enable will allocate a shadow pool of 4MB
> > regardless of the values of sh_min_allocation or
> > shadow_min_acceptable_pages, which means that calls to
> > shadow_alloc_p2m_page can fail even after the check and allocation
> > done just above.
> > 
> > Fix this by always checking that the pool is big enough so the rest of
> > the shadow_init function cannot fail due to lack of pages in the
> > shadow pool. This is relevant to shadow_alloc_p2m_page which requires
> > a minimum amount of shadow_min_acceptable_pages(d) + 1 in the pool.
> > 
> > This allows booting a guest using shadow and more than 6 vCPUs.
> 
> I'm routinely booting 8-vCPU guests without issues.

For me the following simple example with 8 vcpus doesn't work:

# cat test.cfg
name = "test"
type = "hvm"

memory = 256
vcpus = 8
hap = 0
# xl create test.cfg
Parsing config from test.cfg
libxl: error: libxl_create.c:578:libxl__domain_make: domain creation fail: 
Cannot allocate memory
libxl: error: libxl_create.c:975:initiate_domain_create: cannot make domain: -3

> 
> > --- a/xen/arch/x86/mm/shadow/common.c
> > +++ b/xen/arch/x86/mm/shadow/common.c
> > @@ -2705,6 +2705,11 @@ int shadow_enable(struct domain *d, u32 mode)
> >      uint32_t *e;
> >      int rv = 0;
> >      struct p2m_domain *p2m = p2m_get_hostp2m(d);
> > +    /*
> > +     * Required minimum amount of pool pages plus 4MB. This is required so 
> > the
> > +     * calls to p2m_alloc_table and shadow_alloc_p2m_page below don't fail.
> > +     */
> > +    unsigned int min_pages = shadow_min_acceptable_pages(d) + 1024;
> 
> sh_min_allocation() also takes the memory size of the domain into
> account. Aren't you therefore risking to regress larger guests by
> instead using a fixed amount here? The more that ...

shadow_enabled is called by domain_create, and at this point the
memory size of the guest is not yet known AFAICT. I assume the
toolstack will make further hypercalls to set a suitable sized shadow
memory pool after the domain has been created and before populating
the physmap.

Thanks, Roger.

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