Re: [Qemu-devel] [PATCH v2] add -machine mem_merge=on|off option

[Anthony Liguori]

On 06/29/2012 09:00 AM, Luiz Capitulino wrote:
> Allows to disable memory merge support (KSM on Linux), which is enabled
> by default otherwise.
>
> Signed-off-by: Luiz Capitulino<address@hidden>
> ---
>
> o v2: improve manpage description
>
>   exec.c          | 19 ++++++++++++++++---
>   qemu-config.c   |  4 ++++
>   qemu-options.hx |  7 ++++++-
>   3 files changed, 26 insertions(+), 4 deletions(-)
>
> diff --git a/exec.c b/exec.c
> index 8244d54..24dd154 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -2499,6 +2499,19 @@ void qemu_ram_set_idstr(ram_addr_t addr, const char 
> *name, DeviceState *dev)
>       }
>   }
>
> +static int madvise_mergeable(void *addr, size_t len)
> +{
> +    QemuOpts *opts;
> +
> +    opts = qemu_opts_find(qemu_find_opts("machine"), 0);
> +    if (opts&&  !qemu_opt_get_bool(opts, "mem_merge", true)) {
> +        /* disabled by the user */
> +        return 0;
> +    }
> +
> +    return qemu_madvise(addr, len, QEMU_MADV_MERGEABLE);
> +}
> +
>   ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void *host,
>                                      MemoryRegion *mr)
>   {
> @@ -2518,7 +2531,7 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void 
> *host,
>               new_block->host = file_ram_alloc(new_block, size, mem_path);
>               if (!new_block->host) {
>                   new_block->host = qemu_vmalloc(size);
> -                qemu_madvise(new_block->host, size, QEMU_MADV_MERGEABLE);
> +                madvise_mergeable(new_block->host, size);

Just a style thing.  Having a function call that may silently not do what you 
expect it to do is an unfriendly thing.

You should at least name the function something like 'memory_try_enable_merging'

Maybe that's not the best name, but you get the idea at least.

Regards,

Anthony Liguori

>               }
>   #else
>               fprintf(stderr, "-mem-path option unsupported\n");
> @@ -2545,7 +2558,7 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, void 
> *host,
>                   new_block->host = qemu_vmalloc(size);
>               }
>   #endif
> -            qemu_madvise(new_block->host, size, QEMU_MADV_MERGEABLE);
> +            madvise_mergeable(new_block->host, size);
>           }
>       }
>       new_block->length = size;
> @@ -2672,7 +2685,7 @@ void qemu_ram_remap(ram_addr_t addr, ram_addr_t length)
>                               length, addr);
>                       exit(1);
>                   }
> -                qemu_madvise(vaddr, length, QEMU_MADV_MERGEABLE);
> +                madvise_mergeable(vaddr, length);
>               }
>               return;
>           }
> diff --git a/qemu-config.c b/qemu-config.c
> index 5c3296b..e47612e 100644
> --- a/qemu-config.c
> +++ b/qemu-config.c
> @@ -595,6 +595,10 @@ static QemuOptsList qemu_machine_opts = {
>               .name = "dt_compatible",
>               .type = QEMU_OPT_STRING,
>               .help = "Overrides the \"compatible\" property of the dt root 
> node",
> +        }, {
> +            .name = "mem_merge",
> +            .type = QEMU_OPT_BOOL,
> +            .help = "enable/disable memory merge support",
>           },
>           { /* End of list */ }
>       },
> diff --git a/qemu-options.hx b/qemu-options.hx
> index 8b66264..15351a5 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -33,7 +33,8 @@ DEF("machine", HAS_ARG, QEMU_OPTION_machine, \
>       "                property accel=accel1[:accel2[:...]] selects 
> accelerator\n"
>       "                supported accelerators are kvm, xen, tcg (default: 
> tcg)\n"
>       "                kernel_irqchip=on|off controls accelerated irqchip 
> support\n"
> -    "                kvm_shadow_mem=size of KVM shadow MMU\n",
> +    "                kvm_shadow_mem=size of KVM shadow MMU\n"
> +    "                mem_merge=on|off controls memory merge support (default: 
> on)\n",
>       QEMU_ARCH_ALL)
>   STEXI
>   @item -machine address@hidden,address@hidden,...]]
> @@ -50,6 +51,10 @@ to initialize.
>   Enables in-kernel irqchip support for the chosen accelerator when available.
>   @item kvm_shadow_mem=size
>   Defines the size of the KVM shadow MMU.
> address@hidden mem_merge=on|off
> +Enables or disables memory merge support. This feature, when supported by
> +the host, de-duplicates identical memory pages among VMs instances
> +(enabled by default).
>   @end table
>   ETEXI