Re: [Qemu-devel] [PATCH v4 2/4] util/oslib-win32: indicate alignment for qemu_anon_ram_alloc()

[Igor Mammedov <imammedo@redhat.com>]

On Wed,  1 Aug 2018 15:34:42 +0200
David Hildenbrand <david@redhat.com> wrote:

> Let's set the alignment just like for the posix variant. This will
> implicitly set the alignment of the underlying memory region and
> therefore make memory_region_get_alignment(mr) return something > 0 for
> all memory backends applicable to PCDIMM/NVDIMM.
> 
> The allocation granularity is ususally 64k, while the page size is 4k.
> The documentation of VirtualAlloc is not really comprehensible in case
> only MEM_COMMIT is specified without an address. We'll detect the actual
> values and then go for the bigger one. The expection is, that it will
> always be 64k aligned. (The assumption is that MEM_COMMIT does an
> implicit MEM_RESERVE, so the address will always be aligned to the
> allocation granularity. And the allocation granularity is always bigger
> than the page size).
> 
> This will allow us to drop special handling in pc.c for
> memory_region_get_alignment(mr) == 0, as we can then assume that it is
> always set (and AFAICS >= getpagesize()).
> 
> For pc in pc_memory_plug(), under Windows TARGET_PAGE_SIZE == getpagesize(),
> therefore alignment of DIMMs will not change, and therefore also not the
> guest physical memory layout.
> 
> For spapr in spapr_memory_plug(), an alignment of 0 would have been used
> until now. As QEMU_ALIGN_UP will crash with the alignment being 0, this
> never worked, so we don't have to care about compatibility handling.
> 
> Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
> Signed-off-by: David Hildenbrand <david@redhat.com>

Reviewed-by: Igor Mammedov <imammedo@redhat.com>


> ---
>  util/oslib-win32.c | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/util/oslib-win32.c b/util/oslib-win32.c
> index bb5ad28bd3..25dd1595ad 100644
> --- a/util/oslib-win32.c
> +++ b/util/oslib-win32.c
> @@ -67,15 +67,24 @@ void *qemu_memalign(size_t alignment, size_t size)
>      return qemu_oom_check(qemu_try_memalign(alignment, size));
>  }
>  
> +static int get_allocation_granularity(void)
> +{
> +    SYSTEM_INFO system_info;
> +
> +    GetSystemInfo(&system_info);
> +    return system_info.dwAllocationGranularity;
> +}
> +
>  void *qemu_anon_ram_alloc(size_t size, uint64_t *align, bool shared)
>  {
>      void *ptr;
>  
> -    /* FIXME: this is not exactly optimal solution since VirtualAlloc
> -       has 64Kb granularity, but at least it guarantees us that the
> -       memory is page aligned. */
>      ptr = VirtualAlloc(NULL, size, MEM_COMMIT, PAGE_READWRITE);
>      trace_qemu_anon_ram_alloc(size, ptr);
> +
> +    if (ptr && align) {
> +        *align = MAX(get_allocation_granularity(), getpagesize());
> +    }
>      return ptr;
>  }
>