Re: [Xen-devel] [PATCH] x86/Dom0: account for shadow/HAP allocation

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 25/02/15 14:45, Jan Beulich wrote:
> ... when calculating how many pages to allocate fopr Dom0. This is

"fopr" => "for" ?

> basically equivalent to the already present IOMMU related adjustment.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> --- a/xen/arch/x86/domain_build.c
> +++ b/xen/arch/x86/domain_build.c
> @@ -132,6 +132,8 @@ struct vcpu *__init alloc_dom0_vcpu0(str
>  #ifdef CONFIG_SHADOW_PAGING
>  static bool_t __initdata opt_dom0_shadow;
>  boolean_param("dom0_shadow", opt_dom0_shadow);
> +#else
> +#define opt_dom0_shadow 0
>  #endif
>  
>  static char __initdata opt_dom0_ioports_disable[200] = "";
> @@ -203,13 +205,23 @@ static struct page_info * __init alloc_c
>      return page;
>  }
>  
> +static unsigned long __init dom0_paging_pages(const struct domain *d,
> +                                              unsigned long nr_pages)
> +{
> +    /* Copied from: libxl_get_required_shadow_memory() */
> +    unsigned long memkb = nr_pages * (PAGE_SIZE / 1024);
> +
> +    memkb = 4 * (256 * d->max_vcpus + 2 * (memkb / 1024));

I have recently raised a bug against Xapi for similar wrong logic when
calculating the size of the shadow pool.

A per-vcpu reservation of shadow allocation is only needed if shadow
paging is actually in use, and even then should match
shadow_min_acceptable_pages() at 128 pages per vcpu.

If HAP is in use, the only allocations from the shadow pool are for the
EPT/NPT tables (1% of nr_pages), IOMMU tables (another 1% of nr_pages if
in use), and the logdirty radix tree (substantially less than than 1% of
nr_pages).

One could argue that structure such as the vmcs/vmcb should have their
allocations accounted against the domain, in which case a small per-vcpu
component would be appropriate.

However as it currently stands, this calculation wastes 4MB of ram per
vcpu in shadow allocation which is not going to be used.

~Andrew

> +
> +    return ((memkb + 1023) / 1024) << (20 - PAGE_SHIFT);
> +}
> +
>  static unsigned long __init compute_dom0_nr_pages(
>      struct domain *d, struct elf_dom_parms *parms, unsigned long initrd_len)
>  {
>      unsigned long avail = avail_domheap_pages() + initial_images_nrpages();
> -    unsigned long nr_pages = dom0_nrpages;
> -    unsigned long min_pages = dom0_min_nrpages;
> -    unsigned long max_pages = dom0_max_nrpages;
> +    unsigned long nr_pages, min_pages, max_pages;
> +    bool_t need_paging;
>  
>      /* Reserve memory for further dom0 vcpu-struct allocations... */
>      avail -= (d->max_vcpus - 1UL)
> @@ -227,23 +239,37 @@ static unsigned long __init compute_dom0
>              avail -= max_pdx >> s;
>      }
>  
> -    /*
> -     * If domain 0 allocation isn't specified, reserve 1/16th of available
> -     * memory for things like DMA buffers. This reservation is clamped to 
> -     * a maximum of 128MB.
> -     */
> -    if ( nr_pages == 0 )
> -        nr_pages = -min(avail / 16, 128UL << (20 - PAGE_SHIFT));
> +    need_paging = opt_dom0_shadow || (is_pvh_domain(d) && 
> !iommu_hap_pt_share);
> +    for ( ; ; need_paging = 0 )
> +    {
> +        nr_pages = dom0_nrpages;
> +        min_pages = dom0_min_nrpages;
> +        max_pages = dom0_max_nrpages;
> +
> +        /*
> +         * If allocation isn't specified, reserve 1/16th of available memory
> +         * for things like DMA buffers. This reservation is clamped to a
> +         * maximum of 128MB.
> +         */
> +        if ( nr_pages == 0 )
> +            nr_pages = -min(avail / 16, 128UL << (20 - PAGE_SHIFT));
>  
> -    /* Negative memory specification means "all memory - specified amount". 
> */
> -    if ( (long)nr_pages  < 0 ) nr_pages  += avail;
> -    if ( (long)min_pages < 0 ) min_pages += avail;
> -    if ( (long)max_pages < 0 ) max_pages += avail;
> -
> -    /* Clamp dom0 memory according to min/max limits and available memory. */
> -    nr_pages = max(nr_pages, min_pages);
> -    nr_pages = min(nr_pages, max_pages);
> -    nr_pages = min(nr_pages, avail);
> +        /* Negative specification means "all memory - specified amount". */
> +        if ( (long)nr_pages  < 0 ) nr_pages  += avail;
> +        if ( (long)min_pages < 0 ) min_pages += avail;
> +        if ( (long)max_pages < 0 ) max_pages += avail;
> +
> +        /* Clamp according to min/max limits and available memory. */
> +        nr_pages = max(nr_pages, min_pages);
> +        nr_pages = min(nr_pages, max_pages);
> +        nr_pages = min(nr_pages, avail);
> +
> +        if ( !need_paging )
> +            break;
> +
> +        /* Reserve memory for shadow or HAP. */
> +        avail -= dom0_paging_pages(d, nr_pages);
> +    }
>  
>      if ( (parms->p2m_base == UNSET_ADDR) && (dom0_nrpages <= 0) &&
>           ((dom0_min_nrpages <= 0) || (nr_pages > min_pages)) )
> @@ -1287,14 +1313,7 @@ int __init construct_dom0(
>      }
>  
>      if ( is_pvh_domain(d) )
> -    {
> -        unsigned long hap_pages, memkb = nr_pages * (PAGE_SIZE / 1024);
> -
> -        /* Copied from: libxl_get_required_shadow_memory() */
> -        memkb = 4 * (256 * d->max_vcpus + 2 * (memkb / 1024));
> -        hap_pages = ( (memkb + 1023) / 1024) << (20 - PAGE_SHIFT);
> -        hap_set_alloc_for_pvh_dom0(d, hap_pages);
> -    }
> +        hap_set_alloc_for_pvh_dom0(d, dom0_paging_pages(d, nr_pages));
>  
>      /*
>       * We enable paging mode again so guest_physmap_add_page will do the
>
>



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