Re: [PATCH for-4.19] ppc/riscv: fix arch_acquire_resource_check()

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.04.2024 17:34, Roger Pau Monne wrote:
> None of the implementations support set_foreign_p2m_entry() yet, neither they
> have a p2m walk in domain_relinquish_resources() in order to remove the 
> foreign
> mappings from the p2m and thus drop the extra refcounts.

While I don't mind the cod adjustment into the more safe direction, I find
this justification odd: RISC-V has no domain_relinquish_resources() at all
right now, and PPC has it properly as a stub only. Judgement on what there
is (or not) can only be made one non-stub implementations exist.

IOW provided PPC and RISC-V people agree, I'm fine putting this in, but
preferably with an adjusted description. To be honest with how you put it,
it's not even really clear to me what (practical) problem, if any, you're
trying to address.

Jan

> Adjust the arch helpers to return false and introduce a comment that clearly
> states it is not only taking extra refcounts that's needed, but also dropping
> them on domain teardown.
> 
> Fixes: 4988704e00d8 ('xen/riscv: introduce p2m.h')
> Fixes: 4a2f68f90930 ('xen/ppc: Define minimal stub headers required for full 
> build')
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> ---
>  xen/arch/ppc/include/asm/p2m.h   | 7 ++++---
>  xen/arch/riscv/include/asm/p2m.h | 7 ++++---
>  2 files changed, 8 insertions(+), 6 deletions(-)
> 
> diff --git a/xen/arch/ppc/include/asm/p2m.h b/xen/arch/ppc/include/asm/p2m.h
> index 25ba05466853..f144ef8e1a54 100644
> --- a/xen/arch/ppc/include/asm/p2m.h
> +++ b/xen/arch/ppc/include/asm/p2m.h
> @@ -81,10 +81,11 @@ static inline mfn_t gfn_to_mfn(struct domain *d, gfn_t 
> gfn)
>  static inline bool arch_acquire_resource_check(struct domain *d)
>  {
>      /*
> -     * The reference counting of foreign entries in set_foreign_p2m_entry()
> -     * is supported on PPC.
> +     * Requires refcounting the foreign mappings and walking the p2m on
> +     * teardown in order to remove foreign pages from the p2m and drop the
> +     * extra reference counts.
>       */
> -    return true;
> +    return false;
>  }
>  
>  static inline void p2m_altp2m_check(struct vcpu *v, uint16_t idx)
> diff --git a/xen/arch/riscv/include/asm/p2m.h 
> b/xen/arch/riscv/include/asm/p2m.h
> index 87b13f897926..387f372b5d26 100644
> --- a/xen/arch/riscv/include/asm/p2m.h
> +++ b/xen/arch/riscv/include/asm/p2m.h
> @@ -79,10 +79,11 @@ static inline mfn_t gfn_to_mfn(struct domain *d, gfn_t 
> gfn)
>  static inline bool arch_acquire_resource_check(struct domain *d)
>  {
>      /*
> -     * The reference counting of foreign entries in set_foreign_p2m_entry()
> -     * is supported on RISCV.
> +     * Requires refcounting the foreign mappings and walking the p2m on
> +     * teardown in order to remove foreign pages from the p2m and drop the
> +     * extra reference counts.
>       */
> -    return true;
> +    return false;
>  }
>  
>  static inline void p2m_altp2m_check(struct vcpu *v, uint16_t idx)