Re: [Xen-devel] [PATCH v4 2/2] xen/arm: fix mask calculation in pdx_init_mask

[Julien Grall <julien.grall@arm.com>]

Hi,

On 6/17/19 7:50 PM, Stefano Stabellini wrote:
> The mask calculation in pdx_init_mask is wrong when the first bank
> starts at address 0x0. The reason is that pdx_init_mask will do '0 - 1'
> causing an underflow. As a result, the mask becomes 0xffffffffffffffff
> which is the biggest possible mask and ends up causing a significant
> memory waste in the frametable size computation.
> 
> For instance, on platforms that have a low memory bank starting at 0x0
> and a high memory bank, the frametable will end up covering all the
> holes in between.
> 
> The purpose of the mask is to be passed as a parameter to
> pfn_pdx_hole_setup, which based on the mask parameter calculates
> pfn_pdx_hole_shift, pfn_pdx_bottom_mask, etc. which are actually the
> important masks for frametable initialization later on.
> 
> pfn_pdx_hole_setup never compresses addresses below MAX_ORDER bits (1GB
> on ARM). Thus, it is safe to initialize mask passing 1ULL << (MAX_ORDER
> + PAGE_SHIFT) as start address to pdx_init_mask.
> 
> Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>

Reviewed-by: Julien Grall <julien.grall@arm.com>

Ideally, I would like an ack from Andrew or Jan.

> CC: JBeulich@suse.com
> CC: andrew.cooper3@citrix.com
> CC: George.Dunlap@eu.citrix.com
> CC: ian.jackson@eu.citrix.com
> CC: konrad.wilk@oracle.com
> CC: tim@xen.org
> CC: wei.liu2@citrix.com
> ---
> 
> Changes in v4:
> - use uint64_t
> - single line comment code style
> 
> Changes in v3:
> - improve in-code comments
> 
> Unchanged in v3:
> - (u64)1
> 
> Changes in v2:
> - update commit message
> - add in-code comments regarding update sites
> - improve in-code comments
> - move the mask initialization changes to pdx_init_mask
> ---
>   xen/arch/arm/setup.c | 9 ++++++++-
>   xen/common/pdx.c     | 7 ++++++-
>   2 files changed, 14 insertions(+), 2 deletions(-)
> 
> diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
> index b03e7ac330..b0af90e5bf 100644
> --- a/xen/arch/arm/setup.c
> +++ b/xen/arch/arm/setup.c
> @@ -483,7 +483,14 @@ static void __init init_pdx(void)
>   {
>       paddr_t bank_start, bank_size, bank_end;
>   
> -    uint64_t mask = pdx_init_mask(bootinfo.mem.bank[0].start);
> +    /*
> +     * Arm does not have any restrictions on the bits to compress. Pass 0 to
> +     * let the common code further restrict the mask.
> +     *
> +     * If the logic changes in pfn_pdx_hole_setup we might have to
> +     * update this function too.
> +     */
> +    uint64_t mask = pdx_init_mask(0x0);
>       int bank;
>   
>       for ( bank = 0 ; bank < bootinfo.mem.nr_banks; bank++ )
> diff --git a/xen/common/pdx.c b/xen/common/pdx.c
> index 8356f03ce8..9990b94f73 100644
> --- a/xen/common/pdx.c
> +++ b/xen/common/pdx.c
> @@ -50,9 +50,11 @@ static u64 __init fill_mask(u64 mask)
>       return mask;
>   }
>   
> +/* We don't compress the first MAX_ORDER bit of the addresses. */
>   uint64_t __init pdx_init_mask(uint64_t base_addr)
>   {
> -    return fill_mask(base_addr - 1);
> +    return fill_mask(max(base_addr,
> +                         (uint64_t)1 << (MAX_ORDER + PAGE_SHIFT)) - 1);
>   }
>   
>   u64 __init pdx_region_mask(u64 base, u64 len)
> @@ -80,6 +82,9 @@ void __init pfn_pdx_hole_setup(unsigned long mask)
>        * This guarantees that page-pointer arithmetic remains valid within
>        * contiguous aligned ranges of 2^MAX_ORDER pages. Among others, our
>        * buddy allocator relies on this assumption.
> +     *
> +     * If the logic changes here, we might have to update the ARM specific
> +     * init_pdx too.
>        */
>       for ( j = MAX_ORDER-1; ; )
>       {
> 

-- 
Julien Grall

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