[Qemu-devel] [PATCH] linux-user: Tell guest about big host page sizes

[Qemu-devel] [PATCH] linux-user: Tell guest about big host page sizes

[Alexander Graf]

We tell the guest its page size via AUX vectors. The guest process then uses
this page size as information on which boundaries it can mmap() things.

However, if the host has a bigger page size granularity than the guest, it can
not fulfill these mmap() requests - which falls apart when MAP_FIXED is passed
to mmap.

So in that case, let the guest know that we're running on a bigger page size
granularity than the target would require.

This fixes running qemu-ppc (TARGET_PAGE_SIZE=4k) on a 64k page size ppc64 host
for me.

Signed-off-by: Alexander Graf <agraf@suse.de>
---
 linux-user/elfload.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index 0af6292..127c565 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -1545,7 +1545,7 @@ static abi_ulong create_elf_tables(abi_ulong p, int argc, int envc,
     NEW_AUX_ENT(AT_PHDR, (abi_ulong)(info->load_addr + exec->e_phoff));
     NEW_AUX_ENT(AT_PHENT, (abi_ulong)(sizeof (struct elf_phdr)));
     NEW_AUX_ENT(AT_PHNUM, (abi_ulong)(exec->e_phnum));
-    NEW_AUX_ENT(AT_PAGESZ, (abi_ulong)(TARGET_PAGE_SIZE));
+    NEW_AUX_ENT(AT_PAGESZ, (abi_ulong)(MAX(TARGET_PAGE_SIZE, getpagesize())));
     NEW_AUX_ENT(AT_BASE, (abi_ulong)(interp_info ? interp_info->load_addr : 0));
     NEW_AUX_ENT(AT_FLAGS, (abi_ulong)0);
     NEW_AUX_ENT(AT_ENTRY, info->entry);
-- 
1.8.1.4

Re: [Qemu-devel] [PATCH] linux-user: Tell guest about big host page sizes

[Richard Henderson]

On 06/05/2014 02:43 AM, Alexander Graf wrote:
> We tell the guest its page size via AUX vectors. The guest process then uses
> this page size as information on which boundaries it can mmap() things.
> 
> However, if the host has a bigger page size granularity than the guest, it can
> not fulfill these mmap() requests - which falls apart when MAP_FIXED is passed
> to mmap.
> 
> So in that case, let the guest know that we're running on a bigger page size
> granularity than the target would require.
> 
> This fixes running qemu-ppc (TARGET_PAGE_SIZE=4k) on a 64k page size ppc64 host
> for me.
> 
> Signed-off-by: Alexander Graf <agraf@suse.de>
> ---
>  linux-user/elfload.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)

Woot.  Obviously this won't fix all guests, only those that expect a variable
page size to begin.  But it's certainly a step forward.

Reviewed-by: Richard Henderson <rth@twiddle.net>


r~