linux-user: Speed up guest space initialization on 32-bit ARM target

[Qemu-devel] [PATCH 08/10] linux-user: init_guest_space: Don't try to align if we'll reject it

Posted by Luke Shumaker 7 years, 10 months ago

From: Luke Shumaker <lukeshu@parabola.nu>

If the ensure-alignment code gets triggered, then the
"if (host_start && real_start != current_start)" check will always trigger,
so save 2 syscalls and put that check first.

Note that we can't just switch to using MAP_FIXED for that check, because
then we couldn't differentiate between a failure because "there isn't
enough space" and "there isn't enough space *here*".

Signed-off-by: Luke Shumaker <lukeshu@parabola.nu>
---
 linux-user/elfload.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/linux-user/elfload.c b/linux-user/elfload.c
index 5c0ad65611..1b7583d659 100644
--- a/linux-user/elfload.c
+++ b/linux-user/elfload.c
@@ -1871,6 +1871,11 @@ unsigned long init_guest_space(unsigned long host_start,
             return (unsigned long)-1;
         }
 
+        /* Check to see if the address is valid.  */
+        if (host_start && real_start != current_start) {
+            goto try_again;
+        }
+
         /* Ensure the address is properly aligned.  */
         if (real_start & ~qemu_host_page_mask) {
             /* Ideally, we adjust like
@@ -1905,11 +1910,6 @@ unsigned long init_guest_space(unsigned long host_start,
             aligned_start = real_start;
         }
 
-        /* Check to see if the address is valid.  */
-        if (host_start && aligned_start != current_start) {
-            goto try_again;
-        }
-
 #if defined(TARGET_ARM) && !defined(TARGET_AARCH64)
         /* On 32-bit ARM, we need to also be able to map the commpage.  */
         int valid = init_guest_commpage(aligned_start - guest_start,
-- 
2.15.1

Re: [Qemu-devel] [PATCH 08/10] linux-user: init_guest_space: Don't try to align if we'll reject it

Posted by Peter Maydell 7 years, 7 months ago

On 28 December 2017 at 18:08, Luke Shumaker <lukeshu@lukeshu.com> wrote:
> From: Luke Shumaker <lukeshu@parabola.nu>
>
> If the ensure-alignment code gets triggered, then the
> "if (host_start && real_start != current_start)" check will always trigger,
> so save 2 syscalls and put that check first.
>
> Note that we can't just switch to using MAP_FIXED for that check, because
> then we couldn't differentiate between a failure because "there isn't
> enough space" and "there isn't enough space *here*".
>
> Signed-off-by: Luke Shumaker <lukeshu@parabola.nu>
> ---

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>

thanks
-- PMM

Re: [Qemu-devel] [PATCH 08/10] linux-user: init_guest_space: Don't try to align if we'll reject it

Posted by Laurent Vivier 7 years, 7 months ago

Le 28/12/2017 à 19:08, Luke Shumaker a écrit :
> From: Luke Shumaker <lukeshu@parabola.nu>
> 
> If the ensure-alignment code gets triggered, then the
> "if (host_start && real_start != current_start)" check will always trigger,
> so save 2 syscalls and put that check first.
> 
> Note that we can't just switch to using MAP_FIXED for that check, because
> then we couldn't differentiate between a failure because "there isn't
> enough space" and "there isn't enough space *here*".
> 
> Signed-off-by: Luke Shumaker <lukeshu@parabola.nu>
> ---
>  linux-user/elfload.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/linux-user/elfload.c b/linux-user/elfload.c
> index 5c0ad65611..1b7583d659 100644
> --- a/linux-user/elfload.c
> +++ b/linux-user/elfload.c
> @@ -1871,6 +1871,11 @@ unsigned long init_guest_space(unsigned long host_start,
>              return (unsigned long)-1;
>          }
>  
> +        /* Check to see if the address is valid.  */
> +        if (host_start && real_start != current_start) {
> +            goto try_again;
> +        }
> +
>          /* Ensure the address is properly aligned.  */
>          if (real_start & ~qemu_host_page_mask) {
>              /* Ideally, we adjust like
> @@ -1905,11 +1910,6 @@ unsigned long init_guest_space(unsigned long host_start,
>              aligned_start = real_start;
>          }
>  
> -        /* Check to see if the address is valid.  */
> -        if (host_start && aligned_start != current_start) {
> -            goto try_again;
> -        }
> -
>  #if defined(TARGET_ARM) && !defined(TARGET_AARCH64)
>          /* On 32-bit ARM, we need to also be able to map the commpage.  */
>          int valid = init_guest_commpage(aligned_start - guest_start,
> 

Applied to my 'linux-user-for-2.12' branch.

Thanks,
Laurent