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Re: QEMU RAM allocation function fails


From: Dr. David Alan Gilbert
Subject: Re: QEMU RAM allocation function fails
Date: Thu, 5 Nov 2020 19:58:51 +0000
User-agent: Mutt/1.14.6 (2020-07-11)

* Wayne Li (waynli329@gmail.com) wrote:
> Dear QEMU list members,
> 
> We developed a virtual machine that runs on QEMU.  This virtual
> machine is pretty much an emulated P4080 processor with some
> peripherals attached.  Initializing one of these peripherals, i.e. the
> RAM, seems to be having problems.  I use the function
> "memory_region_init_ram" to initialize the RAM and farther down the
> call stack I see that the "qemu_ram_alloc" function returns an address
> of 0 proving the RAM allocation wasn't successful.  Here is the block
> of code in question copied from the file memory.c:
> 
> void memory_region_init_ram_shared_nomigrate(MemoryRegion *mr,
>                                              Object *owner,
>                                              const char *name,
>                                              uint64_t size,
>                                              bool share,
>                                              Error **errp)
> {
>     memory_region_init(mr, owner, name, size);
>     mr->ram = true;
>     mr->terminates = true;
>     mr->destructor = memory_region_destructor_ram;
>     mr->ram_block = qemu_ram_alloc(size, share, mr, errp);
>     mr->dirty_log_mask = tcg_enabled() ? (1 << DIRTY_MEMORY_CODE) : 0;
> }
> 
> Tracing farther into the "qemu_ram_alloc" function reveals that the
> function fails because inside the "qemu_ram_alloc_internal" function
> in file exec.c, the function "ram_block_add" fails.  Interestingly, a
> local_err object is populated here and the msg field in this object is
> populated with the String "cannot set up guest memory 'ram0': Invalid
> argument".  Here is the block of code in question copied from the file
> exec.c:

I'm surprised something didn't print that message out for you - most
callers pass something like &error_fatal at the end and it should print
it I think.

> 
> RAMBlock *qemu_ram_alloc_internal(ram_addr_t size, ram_addr_t max_size,
>                                   void (*resized)(const char*,
>                                                   uint64_t length,
>                                                   void *host),
>                                   void *host, bool resizeable, bool share,
>                                   MemoryRegion *mr, Error **errp)
> {
>     RAMBlock *new_block;
>     Error *local_err = NULL;
> 
>     size = HOST_PAGE_ALIGN(size);
>     max_size = HOST_PAGE_ALIGN(max_size);
>     new_block = g_malloc0(sizeof(*new_block));
>     new_block->mr = mr;
>     new_block->resized = resized;
>     new_block->used_length = size;
>     new_block->max_length = max_size;
>     assert(max_size >= size);
>     new_block->fd = -1;
>     new_block->page_size = getpagesize();
>     new_block->host = host;
>     if (host) {
>         new_block->flags |= RAM_PREALLOC;
>     }
>     if (resizeable) {
>         new_block->flags |= RAM_RESIZEABLE;
>     }
>     ram_block_add(new_block, &local_err, share);
>     if (local_err) {
>         g_free(new_block);
>         error_propagate(errp, local_err);
>         return NULL;
>     }
>     return new_block;
> }
> 
> Anyway, our VM runs fine until it tries to access the RAM region so
> this is a pretty critical problem for us to solve.  Does anyone know
> much about these QEMU functions?  What could be causing these RAM
> initialzation functions to fail in this way?

You're going the right way - keep following it; that 'cannot set up
guest memory' string is only in one place; softmmu/physmem.c - so
the phys_mem_alloc must have failed.  That suggests either your
max_length or your align requirements are wrong; but keep following
it along.

Dave

> -Thanks, Wayne Li
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK




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