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Re: [PATCH v5 04/14] softmmu/memory: Pass ram_flags to qemu_ram_alloc_fr

From: David Hildenbrand
Subject: Re: [PATCH v5 04/14] softmmu/memory: Pass ram_flags to qemu_ram_alloc_from_fd()
Date: Tue, 20 Apr 2021 13:03:11 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1

On 20.04.21 12:45, Philippe Mathieu-Daudé wrote:
On 4/20/21 12:36 PM, David Hildenbrand wrote:
On 20.04.21 12:18, Philippe Mathieu-Daudé wrote:
Hi David,

On 4/20/21 11:52 AM, Philippe Mathieu-Daudé wrote:
On 4/13/21 11:14 AM, David Hildenbrand wrote:
Let's pass in ram flags just like we do with
to clean up and prepare for more flags.

Simplify the documentation of passed ram flags: Looking at our
documentation of RAM_SHARED and RAM_PMEM is sufficient, no need to be

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
   backends/hostmem-memfd.c | 7 ++++---
   hw/misc/ivshmem.c        | 5 ++---
   include/exec/memory.h    | 9 +++------
   include/exec/ram_addr.h  | 6 +-----
   softmmu/memory.c         | 7 +++----
   5 files changed, 13 insertions(+), 21 deletions(-)

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

Actually it would be clearer to define the 0 value, maybe:

#define RAM_NOFLAG     (0 << 0)

This will also turn some code into

ram_flags = backend->share ? RAM_SHARED : RAM_NOFLAG;
ram_flags |= backend->reserve ? RAM_NOFLAG : RAM_NORESERVE;

This is the callee view, withing the API, where you have all
the context.

Looking at other flag users, I barely see any such usage.
be the only ones. That's why I tend to not do it unless there are strong

I'm more concerned about the caller perspective. What means this
magic '0' in the arguments? Then I have to check the prototype.
If the caller uses RAM_NO_FLAGS, I directly understand what is passed.

Yeah, that makes sense. Even cleaner would be using the type system as we do in the kernel, e.g., for GFP flags

typedef unsigned int __bitwise gfp_t;
#define ___GFP_DMA      0x01u
#define __GFP_DMA       ((__force gfp_t)___GFP_DMA)

And passing around gfp_t. Then even using "0" will bail out.

Anyway my comment fits the usual "can be cleaned later" case.

Make sense, thanks.


David / dhildenb

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