Re: [PATCH v4 15/17] xen: mapcache: Remove assumption of RAMBlock with 0 offset

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 30 Apr 2024, Edgar E. Iglesias wrote:
> From: "Edgar E. Iglesias" <edgar.iglesias@xxxxxxx>
> 
> The current mapcache assumes that all memory is mapped
> in a single RAM MR (the first one with offset 0). Remove
> this assumption and propagate the offset to the mapcache
> so it can do reverse mappings (from hostptr -> ram_addr).
> 
> This is in preparation for adding grant mappings.
> 
> Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xxxxxxx>


Looking at xen_remap_bucket, it is only using address_index (without
adding ram_offset) to map foreign memory. From xen_remap_bucket, I would
understand that address_index already includes the ram_offset.

Meaning that if we want to map foreign mapping at address 0x5000, then
address_index would be 0x5000, even if ram_offset is 0x1000.

But then looking xen_ram_addr_from_mapcache_single ram_offset is added
to paddr_index to calculate the physical address. So in that case we
would want address_index to be 0x4000 and ram_offset to be 0x1000. But
xen_remap_bucket would have to sum address_index and ram_offset to map
foreign memory.

So I am a bit confused, did I get it wrong? One more comment below.


> ---
>  hw/xen/xen-mapcache.c         | 25 ++++++++++++++++++-------
>  include/sysemu/xen-mapcache.h |  2 ++
>  system/physmem.c              |  8 ++++----
>  3 files changed, 24 insertions(+), 11 deletions(-)
> 
> diff --git a/hw/xen/xen-mapcache.c b/hw/xen/xen-mapcache.c
> index 09b5f36d9c..1b32d0c003 100644
> --- a/hw/xen/xen-mapcache.c
> +++ b/hw/xen/xen-mapcache.c
> @@ -43,6 +43,9 @@ typedef struct MapCacheEntry {
>  #define XEN_MAPCACHE_ENTRY_DUMMY (1 << 0)
>      uint8_t flags;
>      hwaddr size;
> +
> +    /* Keep ram_addr offset for reverse mappings (hostptr -> ram_addr).  */
> +    ram_addr_t ram_offset;
>      struct MapCacheEntry *next;
>  } MapCacheEntry;
>  
> @@ -165,7 +168,8 @@ static void xen_remap_bucket(MapCache *mc,
>                               void *vaddr,
>                               hwaddr size,
>                               hwaddr address_index,
> -                             bool dummy)
> +                             bool dummy,
> +                             ram_addr_t ram_offset)
>  {
>      uint8_t *vaddr_base;
>      xen_pfn_t *pfns;
> @@ -244,6 +248,7 @@ static void xen_remap_bucket(MapCache *mc,
>      entry->size = size;
>      entry->valid_mapping = g_new0(unsigned long,
>                                    BITS_TO_LONGS(size >> XC_PAGE_SHIFT));
> +    entry->ram_offset = ram_offset;
>  
>      if (dummy) {
>          entry->flags |= XEN_MAPCACHE_ENTRY_DUMMY;
> @@ -264,6 +269,7 @@ static void xen_remap_bucket(MapCache *mc,
>  
>  static uint8_t *xen_map_cache_unlocked(MapCache *mc,
>                                         hwaddr phys_addr, hwaddr size,
> +                                       ram_addr_t ram_offset,
>                                         uint8_t lock, bool dma, bool is_write)
>  {
>      MapCacheEntry *entry, *pentry = NULL,
> @@ -335,14 +341,16 @@ tryagain:
>      if (!entry) {
>          entry = g_new0(MapCacheEntry, 1);
>          pentry->next = entry;
> -        xen_remap_bucket(mc, entry, NULL, cache_size, address_index, dummy);
> +        xen_remap_bucket(mc, entry, NULL, cache_size, address_index, dummy,
> +                         ram_offset);
>      } else if (!entry->lock) {
>          if (!entry->vaddr_base || entry->paddr_index != address_index ||
>                  entry->size != cache_size ||
>                  !test_bits(address_offset >> XC_PAGE_SHIFT,
>                      test_bit_size >> XC_PAGE_SHIFT,
>                      entry->valid_mapping)) {
> -            xen_remap_bucket(mc, entry, NULL, cache_size, address_index, 
> dummy);
> +            xen_remap_bucket(mc, entry, NULL, cache_size, address_index, 
> dummy,
> +                             ram_offset);
>          }
>      }
>  
> @@ -389,13 +397,15 @@ tryagain:
>  
>  uint8_t *xen_map_cache(MemoryRegion *mr,
>                         hwaddr phys_addr, hwaddr size,
> +                       ram_addr_t ram_addr_offset,
>                         uint8_t lock, bool dma,
>                         bool is_write)
>  {
>      uint8_t *p;
>  
>      mapcache_lock(mapcache);
> -    p = xen_map_cache_unlocked(mapcache, phys_addr, size, lock, dma, 
> is_write);
> +    p = xen_map_cache_unlocked(mapcache, phys_addr, size, ram_addr_offset,
> +                               lock, dma, is_write);
>      mapcache_unlock(mapcache);
>      return p;
>  }
> @@ -432,7 +442,8 @@ static ram_addr_t 
> xen_ram_addr_from_mapcache_single(MapCache *mc, void *ptr)
>          raddr = RAM_ADDR_INVALID;
>      } else {
>          raddr = (reventry->paddr_index << mc->bucket_shift) +
> -             ((unsigned long) ptr - (unsigned long) entry->vaddr_base);
> +             ((unsigned long) ptr - (unsigned long) entry->vaddr_base) +
> +             entry->ram_offset;
>      }
>      mapcache_unlock(mc);
>      return raddr;
> @@ -627,8 +638,8 @@ static uint8_t *xen_replace_cache_entry_unlocked(MapCache 
> *mc,
>  
>      trace_xen_replace_cache_entry_dummy(old_phys_addr, new_phys_addr);
>  
> -    xen_remap_bucket(mapcache, entry, entry->vaddr_base,
> -                     cache_size, address_index, false);
> +    xen_remap_bucket(mc, entry, entry->vaddr_base,
> +                     cache_size, address_index, false, entry->ram_offset);
>      if (!test_bits(address_offset >> XC_PAGE_SHIFT,
>                  test_bit_size >> XC_PAGE_SHIFT,
>                  entry->valid_mapping)) {
> diff --git a/include/sysemu/xen-mapcache.h b/include/sysemu/xen-mapcache.h
> index 1ec9e66752..b5e3ea1bc0 100644
> --- a/include/sysemu/xen-mapcache.h
> +++ b/include/sysemu/xen-mapcache.h
> @@ -19,6 +19,7 @@ typedef hwaddr (*phys_offset_to_gaddr_t)(hwaddr phys_offset,
>  void xen_map_cache_init(phys_offset_to_gaddr_t f,
>                          void *opaque);
>  uint8_t *xen_map_cache(MemoryRegion *mr, hwaddr phys_addr, hwaddr size,
> +                       ram_addr_t ram_addr_offset,
>                         uint8_t lock, bool dma,
>                         bool is_write);
>  ram_addr_t xen_ram_addr_from_mapcache(void *ptr);
> @@ -37,6 +38,7 @@ static inline void 
> xen_map_cache_init(phys_offset_to_gaddr_t f,
>  static inline uint8_t *xen_map_cache(MemoryRegion *mr,
>                                       hwaddr phys_addr,
>                                       hwaddr size,
> +                                     ram_addr_t ram_addr_offset,
>                                       uint8_t lock,
>                                       bool dma,
>                                       bool is_write)
> diff --git a/system/physmem.c b/system/physmem.c
> index 1a5ffcba2a..5b16eeccca 100644
> --- a/system/physmem.c
> +++ b/system/physmem.c
> @@ -2228,13 +2228,13 @@ static void *qemu_ram_ptr_length(RAMBlock *block, 
> ram_addr_t addr,
>           * In that case just map the requested area.
>           */
>          if (xen_mr_is_memory(block->mr)) {
> -            return xen_map_cache(block->mr, addr, len, lock, lock,
> -                                 is_write);
> +            return xen_map_cache(block->mr, addr, len, block->offset,
> +                                 lock, lock, is_write);

Have you considered not tracking offset and address separately and
simply do this?

            return xen_map_cache(block->mr, addr + block->offset, len, 
                                 lock, lock, is_write);


>          }
>  
>          block->host = xen_map_cache(block->mr, block->offset,
> -                                    block->max_length, 1,
> -                                    lock, is_write);
> +                                    block->max_length, 0,
> +                                    1, lock, is_write);
>      }
>  
>      return ramblock_ptr(block, addr);
> -- 
> 2.40.1
>