Re: [PATCH V2 3/3] libxl/arm: Add handling of extended regions for DomU

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 10 Sep 2021, Oleksandr Tyshchenko wrote:
> From: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> 
> The extended region (safe range) is a region of guest physical
> address space which is unused and could be safely used to create
> grant/foreign mappings instead of wasting real RAM pages from
> the domain memory for establishing these mappings.
> 
> The extended regions are chosen at the domain creation time and
> advertised to it via "reg" property under hypervisor node in
> the guest device-tree. As region 0 is reserved for grant table
> space (always present), the indexes for extended regions are 1...N.
> If extended regions could not be allocated for some reason,
> Xen doesn't fail and behaves as usual, so only inserts region 0.
> 
> Please note the following limitations:
> - The extended region feature is only supported for 64-bit domain.
> - The ACPI case is not covered.
> 
> ***
> 
> The algorithm to choose extended regions for non-direct mapped
> DomU is simpler in comparison with the algorithm for direct mapped
> Dom0. As we have a lot of unused space above 4GB, provide single
> 1GB-aligned region from the second RAM bank taking into the account
> the maximum supported guest address space size and the amount of
> memory assigned to the guest. The maximum size of the region is 128GB.
> 
> Suggested-by: Julien Grall <jgrall@xxxxxxxxxx>
> Signed-off-by: Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
> ---
> Changes since RFC:
>    - update patch description
>    - drop uneeded "extended-region" DT property
>    - clear reg array in finalise_ext_region() and add a TODO
> ---
>  tools/libs/light/libxl_arm.c | 89 
> +++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 87 insertions(+), 2 deletions(-)
> 
> diff --git a/tools/libs/light/libxl_arm.c b/tools/libs/light/libxl_arm.c
> index e3140a6..8c1d9d7 100644
> --- a/tools/libs/light/libxl_arm.c
> +++ b/tools/libs/light/libxl_arm.c
> @@ -615,9 +615,12 @@ static int make_hypervisor_node(libxl__gc *gc, void *fdt,
>                                "xen,xen");
>      if (res) return res;
>  
> -    /* reg 0 is grant table space */
> +    /*
> +     * reg 0 is a placeholder for grant table space, reg 1 is a placeholder
> +     * for the extended region.
> +     */
>      res = fdt_property_regs(gc, fdt, GUEST_ROOT_ADDRESS_CELLS, 
> GUEST_ROOT_SIZE_CELLS,
> -                            1,GUEST_GNTTAB_BASE, GUEST_GNTTAB_SIZE);
> +                            2, 0, 0, 0, 0);
>      if (res) return res;
>  
>      /*
> @@ -1069,6 +1072,86 @@ static void finalise_one_node(libxl__gc *gc, void 
> *fdt, const char *uname,
>      }
>  }
>  
> +#define ALIGN_UP_TO_GB(x)   (((x) + GB(1) - 1) & (~(GB(1) - 1)))

Why do we need to align it to 1GB when for Dom0 is aligned to 2MB? I
think it makes sense to have the same alignment requirement.


> +
> +#define EXT_REGION_SIZE   GB(128)

The region needs to be described in xen/include/public/arch-arm.h like
GUEST_RAM1_BASE/SIZE.


> +static void finalise_ext_region(libxl__gc *gc, struct xc_dom_image *dom)
> +{
> +    void *fdt = dom->devicetree_blob;
> +    uint32_t regs[(GUEST_ROOT_ADDRESS_CELLS + GUEST_ROOT_SIZE_CELLS) * 2];
> +    be32 *cells = &regs[0];
> +    uint64_t region_size = 0, region_base, bank1end_align, bank1end_max;
> +    uint32_t gpaddr_bits;
> +    libxl_physinfo info;
> +    int offset, rc;
> +
> +    offset = fdt_path_offset(fdt, "/hypervisor");
> +    assert(offset > 0);
> +
> +    if (strcmp(dom->guest_type, "xen-3.0-aarch64")) {
> +        LOG(WARN, "The extended region is only supported for 64-bit guest");
> +        goto out;
> +    }
> +
> +    rc = libxl_get_physinfo(CTX, &info);
> +    assert(!rc);
> +
> +    gpaddr_bits = info.gpaddr_bits;
> +    assert(gpaddr_bits >= 32 && gpaddr_bits <= 48);
> +
> +    /*
> +     * Try to allocate single 1GB-aligned extended region from the second RAM
> +     * bank (above 4GB) taking into the account the maximum supported guest
> +     * address space size and the amount of memory assigned to the guest.
> +     * The maximum size of the region is 128GB.
> +     */
> +    bank1end_max = min(1ULL << gpaddr_bits, GUEST_RAM1_BASE + 
> GUEST_RAM1_SIZE);
> +    bank1end_align = GUEST_RAM1_BASE +
> +        ALIGN_UP_TO_GB((uint64_t)dom->rambank_size[1] << XC_PAGE_SHIFT);
> +
> +    if (bank1end_max <= bank1end_align) {
> +        LOG(WARN, "The extended region cannot be allocated, not enough 
> space");
> +        goto out;
> +    }
> +
> +    if (bank1end_max - bank1end_align > EXT_REGION_SIZE) {
> +        region_base = bank1end_max - EXT_REGION_SIZE;
> +        region_size = EXT_REGION_SIZE;
> +    } else {
> +        region_base = bank1end_align;
> +        region_size = bank1end_max - bank1end_align;
> +    }
> +
> +out:
> +    /*
> +     * The first region for grant table space must be always present.
> +     * If we managed to allocate the extended region then insert it as
> +     * a second region.
> +     * TODO If we failed to allocate the region, we end up inserting
> +     * zero-sized region. This is because we don't know in advance when
> +     * creating hypervisor node whether it would be possible to allocate
> +     * a region, but we have to create a placeholder anyway. The Linux driver
> +     * is able to deal with by checking the region size. We cannot choose
> +     * a region when creating hypervisor node because the guest memory layout
> +     * is not know at that moment (and dom->rambank_size[1] is empty).
> +     * We need to find a way not to expose invalid regions.
> +     */

This is not great -- it would be barely spec compliant. 

When make_hypervisor_node is called we know the max memory of the guest
as build_info.max_memkb should be populate, right?

If so, we could at least detect whether we can have an extended region
(if not caculate the exact start address) from make_hypervisor_node.

total_guest_memory = build_info.max_memkb * 1024;
rambank1_approx = total_guest_memory - GUEST_RAM0_SIZE;
extended_region_size = GUEST_RAM1_SIZE - rambank1_approx;

if (extended_region_size >= MIN_EXT_REGION_SIZE)
    allocate_ext_region


> +    memset(regs, 0, sizeof(regs));
> +    set_range(&cells, GUEST_ROOT_ADDRESS_CELLS, GUEST_ROOT_SIZE_CELLS,
> +              GUEST_GNTTAB_BASE, GUEST_GNTTAB_SIZE);
> +    if (region_size > 0) {

I think we want to check against a minimum amount rather than 0. Maybe 64MB?


> +        LOG(DEBUG, "Extended region: %#"PRIx64"->%#"PRIx64"\n",
> +            region_base, region_base + region_size);
> +
> +        set_range(&cells, GUEST_ROOT_ADDRESS_CELLS, GUEST_ROOT_SIZE_CELLS,
> +                  region_base, region_size);
> +    }
> +
> +    rc = fdt_setprop_inplace(fdt, offset, "reg", regs, sizeof(regs));
> +    assert(!rc);
> +}
> +
>  int libxl__arch_domain_finalise_hw_description(libxl__gc *gc,
>                                                 uint32_t domid,
>                                                 libxl_domain_config *d_config,
> @@ -1109,6 +1192,8 @@ int 
> libxl__arch_domain_finalise_hw_description(libxl__gc *gc,
>  
>      }
>  
> +    finalise_ext_region(gc, dom);
> +
>      for (i = 0; i < GUEST_RAM_BANKS; i++) {
>          const uint64_t size = (uint64_t)dom->rambank_size[i] << 
> XC_PAGE_SHIFT;
>  
> -- 
> 2.7.4
>