Re: [Xen-devel] [PATCH 18/22] xen/arm: p2m: Rework the context switch to another VTTBR in flush_tlb_domain

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 20 Jul 2016, Julien Grall wrote:
> The current implementation of flush_tlb_domain is relying on the domain
> to have a single p2m. With the upcoming feature altp2m, a single domain
> may have different p2m. So we would need to switch to the correct p2m in
> order to flush the TLBs.
> 
> Rather than checking whether the domain is not the current domain, check
> whether the VTTBR is different. The resulting assembly code is much
> smaller: from 38 instructions (+ 2 functions call) to 22 instructions.

That's true but SYSREG reads are more expensive than regular
instructions.


> Signed-off-by: Julien Grall <julien.grall@xxxxxxx>
> ---
>  xen/arch/arm/p2m.c | 18 +++++++++++-------
>  1 file changed, 11 insertions(+), 7 deletions(-)
> 
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index d1b6009..015c1e8 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -151,24 +151,28 @@ void p2m_restore_state(struct vcpu *n)
>  
>  void flush_tlb_domain(struct domain *d)
>  {
> +    struct p2m_domain *p2m = &d->arch.p2m;
>      unsigned long flags = 0;
> +    uint64_t ovttbr;
>  
>      /*
> -     * Update the VTTBR if necessary with the domain d. In this case,
> -     * it's only necessary to flush TLBs on every CPUs with the current VMID
> -     * (our domain).
> +     * ARM only provides an instruction to flush TLBs for the current
> +     * VMID. So switch to the VTTBR of a given P2M if different.
>       */
> -    if ( d != current->domain )
> +    ovttbr = READ_SYSREG64(VTTBR_EL2);
> +    if ( ovttbr != p2m->vttbr )
>      {
>          local_irq_save(flags);
> -        p2m_load_VTTBR(d);
> +        WRITE_SYSREG64(p2m->vttbr, VTTBR_EL2);
> +        isb();
>      }
>  
>      flush_tlb();
>  
> -    if ( d != current->domain )
> +    if ( ovttbr != READ_SYSREG64(VTTBR_EL2) )

You should be able to remove this second SYSREG read and optimize the
code further.


>      {
> -        p2m_load_VTTBR(current->domain);
> +        WRITE_SYSREG64(ovttbr, VTTBR_EL2);
> +        isb();
>          local_irq_restore(flags);
>      }
>  }
> -- 
> 1.9.1
> 

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