Re: [PATCH for-4.12 v2 17/17] xen/arm: Track page accessed between batch of Set/Way operations

[Stefano Stabellini <sstabellini@kernel.org>]

On Tue, 4 Dec 2018, Julien Grall wrote:
> At the moment, the implementation of Set/Way operations will go through
> all the entries of the guest P2M and flush them. However, this is very
> expensive and may render unusable a guest OS using them.
> 
> For instance, Linux 32-bit will use Set/Way operations during secondary
> CPU bring-up. As the implementation is really expensive, it may be possible
> to hit the CPU bring-up timeout.
> 
> To limit the Set/Way impact, we track what pages has been of the guest
> has been accessed between batch of Set/Way operations. This is done
> using bit[0] (aka valid bit) of the P2M entry.
> 
> This patch adds a new per-arch helper is introduced to perform actions just
> before the guest is first unpaused. This will be used to invalidate the
> P2M to track access from the start of the guest.
> 
> Signed-off-by: Julien Grall <julien.grall@arm.com>
> 
> ---
> 
> While we can spread d->creation_finished all over the code, the per-arch
> helper to perform actions just before the guest is first unpaused can
> bring a lot of benefit for both architecture. For instance, on Arm, the
> flush to the instruction cache could be delayed until the domain is
> first run. This would improve greatly the performance of creating guest.
> 
> I am still doing the benchmark whether having a command line option is
> worth it. I will provide numbers as soon as I have them.
> 
> Cc: Stefano Stabellini <sstabellini@kernel.org>
> Cc: Julien Grall <julien.grall@arm.com>
> Cc: Andrew Cooper <andrew.cooper3@citrix.com>
> Cc: George Dunlap <George.Dunlap@eu.citrix.com>
> Cc: Ian Jackson <ian.jackson@eu.citrix.com>
> Cc: Jan Beulich <jbeulich@suse.com>
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
> Cc: Tim Deegan <tim@xen.org>
> Cc: Wei Liu <wei.liu2@citrix.com>
> ---
>  xen/arch/arm/domain.c     | 14 ++++++++++++++
>  xen/arch/arm/p2m.c        | 30 ++++++++++++++++++++++++++++--
>  xen/arch/x86/domain.c     |  4 ++++
>  xen/common/domain.c       |  5 ++++-
>  xen/include/asm-arm/p2m.h |  2 ++
>  xen/include/xen/domain.h  |  2 ++
>  6 files changed, 54 insertions(+), 3 deletions(-)
> 
> diff --git a/xen/arch/arm/domain.c b/xen/arch/arm/domain.c
> index 1d926dcb29..41f101746e 100644
> --- a/xen/arch/arm/domain.c
> +++ b/xen/arch/arm/domain.c
> @@ -767,6 +767,20 @@ int arch_domain_soft_reset(struct domain *d)
>      return -ENOSYS;
>  }
>  
> +void arch_domain_creation_finished(struct domain *d)
> +{
> +    /*
> +     * To avoid flushing the whole guest RAM on the first Set/Way, we
> +     * invalidate the P2M to track what has been accessed.
> +     *
> +     * This is only turned when IOMMU is not used or the page-table are
> +     * not shared because bit[0] (e.g valid bit) unset will result
> +     * IOMMU fault that could be not fixed-up.
> +     */
> +    if ( !iommu_use_hap_pt(d) )
> +        p2m_invalidate_root(p2m_get_hostp2m(d));
> +}
> +
>  static int is_guest_pv32_psr(uint32_t psr)
>  {
>      switch (psr & PSR_MODE_MASK)
> diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> index 8ee6ff7bd7..44ea3580cf 100644
> --- a/xen/arch/arm/p2m.c
> +++ b/xen/arch/arm/p2m.c
> @@ -1079,6 +1079,22 @@ static void p2m_invalidate_table(struct p2m_domain *p2m, mfn_t mfn)
>  }
>  
>  /*
> + * Invalidate all entries in the root page-tables. This is
> + * useful to get fault on entry and do an action.
> + */
> +void p2m_invalidate_root(struct p2m_domain *p2m)
> +{
> +    unsigned int i;
> +
> +    p2m_write_lock(p2m);
> +
> +    for ( i = 0; i < P2M_ROOT_LEVEL; i++ )
> +        p2m_invalidate_table(p2m, page_to_mfn(p2m->root + i));
> +
> +    p2m_write_unlock(p2m);
> +}
> +
> +/*
>   * Resolve any translation fault due to change in the p2m. This
>   * includes break-before-make and valid bit cleared.
>   */
> @@ -1587,15 +1603,18 @@ int p2m_cache_flush_range(struct domain *d, gfn_t *pstart, gfn_t end)
>           */
>          if ( gfn_eq(start, next_block_gfn) )
>          {
> -            mfn = p2m_get_entry(p2m, start, &t, NULL, &order, NULL);
> +            bool valid;
> +
> +            mfn = p2m_get_entry(p2m, start, &t, NULL, &order, &valid);
>              next_block_gfn = gfn_next_boundary(start, order);
>  
>              /*
>               * The following regions can be skipped:
>               *      - Hole
>               *      - non-RAM
> +             *      - block with valid bit (bit[0]) unset
>               */
> -            if ( mfn_eq(mfn, INVALID_MFN) || !p2m_is_any_ram(t) )
> +            if ( mfn_eq(mfn, INVALID_MFN) || !p2m_is_any_ram(t) || !valid )
>              {
>                  count++;
>                  start = next_block_gfn;
> @@ -1629,6 +1648,7 @@ int p2m_cache_flush_range(struct domain *d, gfn_t *pstart, gfn_t end)
>   */
>  void p2m_flush_vm(struct vcpu *v)
>  {
> +    struct p2m_domain *p2m = p2m_get_hostp2m(v->domain);
>      int rc;
>      gfn_t start = _gfn(0);
>  
> @@ -1648,6 +1668,12 @@ void p2m_flush_vm(struct vcpu *v)
>                  "P2M has not been correctly cleaned (rc = %d)\n",
>                  rc);
>  
> +    /*
> +     * Invalidate the p2m to track which page was modified by the guest
> +     * between call of p2m_flush_vm().
> +     */
> +    p2m_invalidate_root(p2m);

Does this mean that we are invalidating the p2m once more than
necessary, when the caches are finally enabled in Linux? Could that be
avoided by passing an additional argument to p2m_flush_vm?
Is this optimization I am suggesting unimportant?


>      v->arch.need_flush_to_ram = false;
>  }
>  
> diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
> index b4d59487ad..d28e3f9b15 100644
> --- a/xen/arch/x86/domain.c
> +++ b/xen/arch/x86/domain.c
> @@ -762,6 +762,10 @@ int arch_domain_soft_reset(struct domain *d)
>      return ret;
>  }
>  
> +void arch_domain_creation_finished(struct domain *d)
> +{
> +}
> +
>  /*
>   * These are the masks of CR4 bits (subject to hardware availability) which a
>   * PV guest may not legitimiately attempt to modify.
> diff --git a/xen/common/domain.c b/xen/common/domain.c
> index 78cc5249e8..c623daec56 100644
> --- a/xen/common/domain.c
> +++ b/xen/common/domain.c
> @@ -1116,8 +1116,11 @@ int domain_unpause_by_systemcontroller(struct domain *d)
>       * Creation is considered finished when the controller reference count
>       * first drops to 0.
>       */
> -    if ( new == 0 )
> +    if ( new == 0 && !d->creation_finished )
> +    {
>          d->creation_finished = true;
> +        arch_domain_creation_finished(d);
> +    }
>  
>      domain_unpause(d);
>  
> diff --git a/xen/include/asm-arm/p2m.h b/xen/include/asm-arm/p2m.h
> index 79abcb5a63..01cd3ee4b5 100644
> --- a/xen/include/asm-arm/p2m.h
> +++ b/xen/include/asm-arm/p2m.h
> @@ -231,6 +231,8 @@ int p2m_set_entry(struct p2m_domain *p2m,
>  
>  bool p2m_resolve_translation_fault(struct domain *d, gfn_t gfn);
>  
> +void p2m_invalidate_root(struct p2m_domain *p2m);
> +
>  /*
>   * Clean & invalidate caches corresponding to a region [start,end) of guest
>   * address space.
> diff --git a/xen/include/xen/domain.h b/xen/include/xen/domain.h
> index 33e41486cb..d1bfc82f57 100644
> --- a/xen/include/xen/domain.h
> +++ b/xen/include/xen/domain.h
> @@ -70,6 +70,8 @@ void arch_domain_unpause(struct domain *d);
>  
>  int arch_domain_soft_reset(struct domain *d);
>  
> +void arch_domain_creation_finished(struct domain *d);
> +
>  void arch_p2m_set_access_required(struct domain *d, bool access_required);
>  
>  int arch_set_info_guest(struct vcpu *, vcpu_guest_context_u);
> -- 
> 2.11.0
> 

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