[PING^2] linux-user: Passthrough MADV_DONTNEED for certain file mappings

[Ilya Leoshkevich <iii@linux.ibm.com>]

On Fri, 2022-07-01 at 15:52 +0200, Ilya Leoshkevich wrote:
> This is a follow-up for commit 892a4f6a750a ("linux-user: Add partial
> support for MADV_DONTNEED"), which added passthrough for anonymous
> mappings. File mappings can be handled in a similar manner.
> 
> In order to do that, mark pages, for which mmap() was passed through,
> with PAGE_PASSTHROUGH, and then allow madvise() passthrough for these
> pages as well.
> 
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>  include/exec/cpu-all.h |  6 ++++++
>  linux-user/mmap.c      | 25 +++++++++++++++++++++----
>  2 files changed, 27 insertions(+), 4 deletions(-)
> 
> diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h
> index f5bda2c3ca..fbdbc0fdec 100644
> --- a/include/exec/cpu-all.h
> +++ b/include/exec/cpu-all.h
> @@ -262,6 +262,12 @@ extern const TargetPageBits target_page;
>  #define PAGE_TARGET_1  0x0200
>  #define PAGE_TARGET_2  0x0400
>  
> +/*
> + * For linux-user, indicates that the page is mapped with the same
> semantics
> + * in both guest and host.
> + */
> +#define PAGE_PASSTHROUGH 0x0080
> +
>  #if defined(CONFIG_USER_ONLY)
>  void page_dump(FILE *f);
>  
> diff --git a/linux-user/mmap.c b/linux-user/mmap.c
> index 4e7a6be6ee..58622a0c15 100644
> --- a/linux-user/mmap.c
> +++ b/linux-user/mmap.c
> @@ -424,7 +424,8 @@ abi_ulong mmap_find_vma(abi_ulong start,
> abi_ulong size, abi_ulong align)
>  abi_long target_mmap(abi_ulong start, abi_ulong len, int
> target_prot,
>                       int flags, int fd, abi_ulong offset)
>  {
> -    abi_ulong ret, end, real_start, real_end, retaddr, host_offset,
> host_len;
> +    abi_ulong ret, end, real_start, real_end, retaddr, host_offset,
> host_len,
> +              passthrough_start = -1, passthrough_end = -1;
>      int page_flags, host_prot;
>  
>      mmap_lock();
> @@ -537,6 +538,8 @@ abi_long target_mmap(abi_ulong start, abi_ulong
> len, int target_prot,
>              host_start += offset - host_offset;
>          }
>          start = h2g(host_start);
> +        passthrough_start = start;
> +        passthrough_end = start + len;
>      } else {
>          if (start & ~TARGET_PAGE_MASK) {
>              errno = EINVAL;
> @@ -619,6 +622,8 @@ abi_long target_mmap(abi_ulong start, abi_ulong
> len, int target_prot,
>                       host_prot, flags, fd, offset1);
>              if (p == MAP_FAILED)
>                  goto fail;
> +            passthrough_start = real_start;
> +            passthrough_end = real_end;
>          }
>      }
>   the_end1:
> @@ -626,7 +631,18 @@ abi_long target_mmap(abi_ulong start, abi_ulong
> len, int target_prot,
>          page_flags |= PAGE_ANON;
>      }
>      page_flags |= PAGE_RESET;
> -    page_set_flags(start, start + len, page_flags);
> +    if (passthrough_start == passthrough_end) {
> +        page_set_flags(start, start + len, page_flags);
> +    } else {
> +        if (start != passthrough_start) {
> +            page_set_flags(start, passthrough_start, page_flags);
> +        }
> +        page_set_flags(passthrough_start, passthrough_end,
> +                       page_flags | PAGE_PASSTHROUGH);
> +        if (passthrough_end != start + len) {
> +            page_set_flags(passthrough_end, start + len,
> page_flags);
> +        }
> +    }
>   the_end:
>      trace_target_mmap_complete(start);
>      if (qemu_loglevel_mask(CPU_LOG_PAGE)) {
> @@ -845,7 +861,7 @@ static bool
> can_passthrough_madv_dontneed(abi_ulong start, abi_ulong end)
>      }
>  
>      for (addr = start; addr < end; addr += TARGET_PAGE_SIZE) {
> -        if (!(page_get_flags(addr) & PAGE_ANON)) {
> +        if (!(page_get_flags(addr) & (PAGE_ANON |
> PAGE_PASSTHROUGH))) {
>              return false;
>          }
>      }
> @@ -888,7 +904,8 @@ abi_long target_madvise(abi_ulong start,
> abi_ulong len_in, int advice)
>       *
>       * This breaks MADV_DONTNEED, completely implementing which is
> quite
>       * complicated. However, there is one low-hanging fruit: host-
> page-aligned
> -     * anonymous mappings. In this case passthrough is safe, so do
> it.
> +     * anonymous mappings or mappings that are known to have the
> same semantics
> +     * in the host and the guest. In this case passthrough is safe,
> so do it.
>       */
>      mmap_lock();
>      if ((advice & MADV_DONTNEED) &&

Ping^2:

https://patchew.org/QEMU/20220701135207.2710488-1-iii@linux.ibm.com/

Is there still a chance that this can get into QEMU 7.1?