Re: [PATCH v2 3/7] xen/p2m: put reference for level 2 superpage

[Michal Orzel <michal.orzel@xxxxxxx>]

Hi Luca,

On 15/05/2024 16:26, Luca Fancellu wrote:
> 
> 
> From: Penny Zheng <Penny.Zheng@xxxxxxx>
> 
> We are doing foreign memory mapping for static shared memory, and
> there is a great possibility that it could be super mapped.
> But today, p2m_put_l3_page could not handle superpages.
s/could/can

> 
> This commits implements a new function p2m_put_l2_superpage to handle
> 2MB superpages, specifically for helping put extra references for
> foreign superpages.
> 
> Modify relinquish_p2m_mapping as well to take into account preemption
> when type is foreign memory and order is above 9 (2MB).
I don't see the reasoning why we don't handle 1GB super pages. It would be 
beneficial
to include such in commit msg as well as in the code.

> 
> Signed-off-by: Penny Zheng <penny.zheng@xxxxxxx>
> Signed-off-by: Luca Fancellu <luca.fancellu@xxxxxxx>
> ---
> v2:
>  - Do not handle 1GB super page as there might be some issue where
>    a lot of calls to put_page(...) might be issued which could lead
>    to free memory that is a long operation.
> v1:
>  - patch from 
> https://patchwork.kernel.org/project/xen-devel/patch/20231206090623.1932275-9-Penny.Zheng@xxxxxxx/
> ---
>  xen/arch/arm/mmu/p2m.c | 57 +++++++++++++++++++++++++++++-------------
>  1 file changed, 40 insertions(+), 17 deletions(-)
> 
> diff --git a/xen/arch/arm/mmu/p2m.c b/xen/arch/arm/mmu/p2m.c
> index 41fcca011cf4..29fdb3b87dd0 100644
> --- a/xen/arch/arm/mmu/p2m.c
> +++ b/xen/arch/arm/mmu/p2m.c
> @@ -753,17 +753,9 @@ static int p2m_mem_access_radix_set(struct p2m_domain 
> *p2m, gfn_t gfn,
>      return rc;
>  }
> 
> -/*
> - * Put any references on the single 4K page referenced by pte.
> - * TODO: Handle superpages, for now we only take special references for leaf
> - * pages (specifically foreign ones, which can't be super mapped today).
> - */
> -static void p2m_put_l3_page(const lpae_t pte)
> +/* Put any references on the single 4K page referenced by mfn. */
> +static void p2m_put_l3_page(mfn_t mfn, p2m_type_t type)
>  {
> -    mfn_t mfn = lpae_get_mfn(pte);
> -
> -    ASSERT(p2m_is_valid(pte));
> -
>      /*
>       * TODO: Handle other p2m types
>       *
> @@ -771,16 +763,44 @@ static void p2m_put_l3_page(const lpae_t pte)
>       * flush the TLBs if the page is reallocated before the end of
>       * this loop.
>       */
> -    if ( p2m_is_foreign(pte.p2m.type) )
> +    if ( p2m_is_foreign(type) )
>      {
>          ASSERT(mfn_valid(mfn));
>          put_page(mfn_to_page(mfn));
>      }
>      /* Detect the xenheap page and mark the stored GFN as invalid. */
> -    else if ( p2m_is_ram(pte.p2m.type) && is_xen_heap_mfn(mfn) )
> +    else if ( p2m_is_ram(type) && is_xen_heap_mfn(mfn) )
>          page_set_xenheap_gfn(mfn_to_page(mfn), INVALID_GFN);
>  }
> 
> +/* Put any references on the superpage referenced by mfn. */
> +static void p2m_put_l2_superpage(mfn_t mfn, p2m_type_t type)
> +{
> +    unsigned int i;
> +    unsigned int level_order = XEN_PT_LEVEL_ORDER(3);
If we know the third level order is always 0 no matter the page shift, does it 
make sense to have this variable?

> +
> +    for ( i = 0; i < XEN_PT_LPAE_ENTRIES; i++ )
> +    {
> +        p2m_put_l3_page(mfn, type);
> +
> +        mfn = mfn_add(mfn, 1 << level_order);
> +    }
> +}
> +
> +/* Put any references on the page referenced by pte. */
> +static void p2m_put_page(const lpae_t pte, unsigned int level)
> +{
> +    mfn_t mfn = lpae_get_mfn(pte);
> +
> +    ASSERT(p2m_is_valid(pte));
> +
> +    /* We have a second level 2M superpage */
> +    if ( p2m_is_superpage(pte, level) && (level == 2) )
> +        return p2m_put_l2_superpage(mfn, pte.p2m.type);
> +    else if ( level == 3 )
> +        return p2m_put_l3_page(mfn, pte.p2m.type);
> +}
> +
>  /* Free lpae sub-tree behind an entry */
>  static void p2m_free_entry(struct p2m_domain *p2m,
>                             lpae_t entry, unsigned int level)
> @@ -809,9 +829,10 @@ static void p2m_free_entry(struct p2m_domain *p2m,
>  #endif
> 
>          p2m->stats.mappings[level]--;
> -        /* Nothing to do if the entry is a super-page. */
> -        if ( level == 3 )
> -            p2m_put_l3_page(entry);
> +        /* TODO: Currently we don't handle 1GB super-page. */
As a future reader of the code it would be beneficial to say why.

> +        if ( level >= 2 )
> +            p2m_put_page(entry, level);
> +
>          return;
>      }
> 
> @@ -1558,9 +1579,11 @@ int relinquish_p2m_mapping(struct domain *d)
> 
>          count++;
>          /*
> -         * Arbitrarily preempt every 512 iterations.
> +         * Arbitrarily preempt every 512 iterations or when type is foreign
> +         * mapping and the order is above 9 (2MB).
>           */
> -        if ( !(count % 512) && hypercall_preempt_check() )
> +        if ( (!(count % 512) || (p2m_is_foreign(t) && (order > 9))) &&
Instead of (order > 9), use XEN_PT_LEVEL_ORDER(2)

> +             hypercall_preempt_check() )
>          {
>              rc = -ERESTART;
>              break;
> --
> 2.34.1
> 

~Michal