Re: [Xen-devel] [PATCH v2 3/3] xen/arm: fix mask calculation in pdx_init_mask

[Julien Grall <julien.grall@xxxxxxx>]

Hi Stefano,

On 5/8/19 11:47 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@xxxxxxxxxx>
> CC: JBeulich@xxxxxxxx
> CC: andrew.cooper3@xxxxxxxxxx
> CC: George.Dunlap@xxxxxxxxxxxxx
> CC: ian.jackson@xxxxxxxxxxxxx
> CC: konrad.wilk@xxxxxxxxxx
> CC: tim@xxxxxxx
> CC: wei.liu2@xxxxxxxxxx
> ---
>
> 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     | 8 +++++++-
>   2 files changed, 15 insertions(+), 2 deletions(-)
>
> diff --git a/xen/arch/arm/setup.c b/xen/arch/arm/setup.c
> index ccb0f181ea..afaafe7b84 100644
> --- a/xen/arch/arm/setup.c
> +++ b/xen/arch/arm/setup.c
> @@ -482,7 +482,14 @@ static void __init init_pdx(void)
>   {
>       paddr_t bank_start, bank_size, bank_end;
>   
> -    u64 mask = pdx_init_mask(bootinfo.mem.bank[0].start);
> +    /*
> +     * Pass 0x0 to pdx_init_mask to get a mask initialized with the
> +     * first to 1<<MAX_ORDER pages of RAM left uncompressed.
What matter is Arm doesn't have any specific restriction on the 
compression, but the common code may have. So how about something:
"Arm does not have any restriction on the bits to compress. Pass 0 to 
let the common code to further restrict".
> +     *
> +     * If the logic changes in pfn_pdx_hole_setup we might have to
> +     * update this function too.
> +     */ > +    u64 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 bb7e437049..268d6f7ec3 100644
> --- a/xen/common/pdx.c
> +++ b/xen/common/pdx.c
> @@ -50,9 +50,13 @@ static u64 __init fill_mask(u64 mask)
>       return mask;
>   }
>   
> +/*
> + * We always map the first 1<<MAX_ORDER pages of RAM, hence, they
I thought I already pointed out in the previous version. At least on 
Arm, we never map the first 1 << MAX_ORDER of RAM. Instead what you want 
to say is that we don't compress the first N bits of the address.
> + * are left uncompressed.
> + */
Cheers,

--
Julien Grall

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel