Re: [PATCH V3 (resend) 09/19] x86/domain_page: Remove the fast paths when mfn is not in the directmap

["Roger Pau Monné" <roger.pau@citrix.com>]

On Mon, May 13, 2024 at 01:40:36PM +0000, Elias El Yandouzi wrote:
> From: Hongyan Xia <hongyxia@amazon.com>
> 
> When mfn is not in direct map, never use mfn_to_virt for any mappings.
> 
> We replace mfn_x(mfn) <= PFN_DOWN(__pa(HYPERVISOR_VIRT_END - 1)) with
> arch_mfns_in_direct_map(mfn, 1) because these two are equivalent. The
> extra comparison in arch_mfns_in_direct_map() looks different but because
> DIRECTMAP_VIRT_END is always higher, it does not make any difference.
> 
> Lastly, domain_page_map_to_mfn() needs to gain to a special case for
> the PMAP.
> 
> Signed-off-by: Hongyan Xia <hongyxia@amazon.com>
> Signed-off-by: Julien Grall <jgrall@amazon.com>
> 
> ----
> 
>     Changes since Hongyan's version:
>         * arch_mfn_in_direct_map() was renamed to arch_mfns_in_directmap()
>         * add a special case for the PMAP in domain_page_map_to_mfn()
> 
> diff --git a/xen/arch/x86/domain_page.c b/xen/arch/x86/domain_page.c
> index 55e337aaf7..89caefc8a2 100644
> --- a/xen/arch/x86/domain_page.c
> +++ b/xen/arch/x86/domain_page.c
> @@ -14,8 +14,10 @@
>  #include <xen/sched.h>
>  #include <xen/vmap.h>
>  #include <asm/current.h>
> +#include <asm/fixmap.h>
>  #include <asm/flushtlb.h>
>  #include <asm/hardirq.h>
> +#include <asm/pmap.h>
>  #include <asm/setup.h>
>  
>  static DEFINE_PER_CPU(struct vcpu *, override);
> @@ -35,10 +37,11 @@ static inline struct vcpu *mapcache_current_vcpu(void)
>      /*
>       * When using efi runtime page tables, we have the equivalent of the idle
>       * domain's page tables but current may point at another domain's VCPU.
> -     * Return NULL as though current is not properly set up yet.
> +     * Return the idle domains's vcpu on that core because the efi per-domain
> +     * region (where the mapcache is) is in-sync with the idle domain.
>       */
>      if ( efi_rs_using_pgtables() )
> -        return NULL;
> +        return idle_vcpu[smp_processor_id()];

There's already an existing instance of idle_vcpu[smp_processor_id()]
down in the function, it might make sense to put this in a local
variable.

>  
>      /*
>       * If guest_table is NULL, and we are running a paravirtualised guest,
> @@ -77,18 +80,24 @@ void *map_domain_page(mfn_t mfn)
>      struct vcpu_maphash_entry *hashent;
>  
>  #ifdef NDEBUG
> -    if ( mfn_x(mfn) <= PFN_DOWN(__pa(HYPERVISOR_VIRT_END - 1)) )
> +    if ( arch_mfns_in_directmap(mfn_x(mfn), 1) )
>          return mfn_to_virt(mfn_x(mfn));
>  #endif
>  
>      v = mapcache_current_vcpu();
> -    if ( !v )
> -        return mfn_to_virt(mfn_x(mfn));
> +    if ( !v || !v->domain->arch.mapcache.inuse )
> +    {
> +        if ( arch_mfns_in_directmap(mfn_x(mfn), 1) )
> +            return mfn_to_virt(mfn_x(mfn));
> +        else
> +        {
> +            BUG_ON(system_state >= SYS_STATE_smp_boot);
> +            return pmap_map(mfn);
> +        }
> +    }
>  
>      dcache = &v->domain->arch.mapcache;
>      vcache = &v->arch.mapcache;
> -    if ( !dcache->inuse )
> -        return mfn_to_virt(mfn_x(mfn));
>  
>      perfc_incr(map_domain_page_count);
>  
> @@ -184,6 +193,12 @@ void unmap_domain_page(const void *ptr)
>      if ( !va || va >= DIRECTMAP_VIRT_START )
>          return;
>  
> +    if ( va >= FIXADDR_START && va < FIXADDR_TOP )

This should be a fixmap helper IMO. virt_is_fixmap(addr) or similar.
There's already an existing instance in virt_to_fix().

> +    {
> +        pmap_unmap((void *)ptr);
> +        return;
> +    }
> +
>      ASSERT(va >= MAPCACHE_VIRT_START && va < MAPCACHE_VIRT_END);
>  
>      v = mapcache_current_vcpu();
> @@ -237,7 +252,7 @@ int mapcache_domain_init(struct domain *d)
>      unsigned int bitmap_pages;
>  
>  #ifdef NDEBUG
> -    if ( !mem_hotplug && max_page <= PFN_DOWN(__pa(HYPERVISOR_VIRT_END - 1)) )
> +    if ( !mem_hotplug && arch_mfn_in_directmap(0, max_page) )
>          return 0;
>  #endif
>  
> @@ -308,7 +323,7 @@ void *map_domain_page_global(mfn_t mfn)
>              local_irq_is_enabled()));
>  
>  #ifdef NDEBUG
> -    if ( mfn_x(mfn) <= PFN_DOWN(__pa(HYPERVISOR_VIRT_END - 1)) )
> +    if ( arch_mfn_in_directmap(mfn_x(mfn, 1)) )
>          return mfn_to_virt(mfn_x(mfn));
>  #endif
>  
> @@ -335,6 +350,23 @@ mfn_t domain_page_map_to_mfn(const void *ptr)
>      if ( va >= DIRECTMAP_VIRT_START )
>          return _mfn(virt_to_mfn(ptr));
>  
> +    /*
> +     * The fixmap is stealing the top-end of the VMAP. So the check for
> +     * the PMAP *must* happen first.
> +     *
> +     * Also, the fixmap translate a slot to an address backwards. The
> +     * logic will rely on it to avoid any complexity. So check at
> +     * compile time this will always hold.
> +    */
> +    BUILD_BUG_ON(fix_to_virt(FIX_PMAP_BEGIN) < fix_to_virt(FIX_PMAP_END));
> +
> +    if ( ((unsigned long)fix_to_virt(FIX_PMAP_END) <= va) &&
> +         ((va & PAGE_MASK) <= (unsigned long)fix_to_virt(FIX_PMAP_BEGIN)) )
> +    {

Can we place this as some kind of helper in fixmap.h?

It's already quite ugly, and could be useful in other places.

bool virt_in_fixmap_range(addr, start idx, end idx)

Or something similar.

Thanks, Roger.