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Re: [Qemu-devel] Re: [PATCH 2 of 5] add can_dma/post_dma for direct IO

From: Andreas Färber
Subject: Re: [Qemu-devel] Re: [PATCH 2 of 5] add can_dma/post_dma for direct IO
Date: Sat, 13 Dec 2008 18:54:21 +0100

Am 13.12.2008 um 17:53 schrieb Andrea Arcangeli:

On Sat, Dec 13, 2008 at 10:46:49AM -0600, Anthony Liguori wrote:
Andrea Arcangeli wrote:
On Fri, Dec 12, 2008 at 01:15:13PM -0600, Anthony Liguori wrote:

void *cpu_physical_memory_map(target_phys_addr_t addr, ram_addr_t size,
int is_write);

Just a side note (doesn't mean I agree with the above), it doesn't
make sense to use an ram_addr_t size when you return a 32bit address
on 32bit qemu build.

size_t is completely wrong for 64-bit targets on 32-bit hosts. ram_addr_t is the type we use for guest ram size. It's 64-bit all of the time simply because it's easier to do that and we decided that the little bit of wasted
space/computations were not a problem.

Not sure why you think I'm suggesting you to use size_t. I'm just
trying to tell you that if you insist in this
64bit-guest-on-32bit-host-is-dead-and-obsolete-to-support (i.e. if you
pass a ram_addr_t size to cpu_physical_memory_map) you've at least to
return ram_addr_t too). 'void *' is like size_t so the above API
getting ram_addr_t length and returning 'void *', can't possibly be

Since memory is allocated on the host with the host's limits, void* seems correct.

That you can't alloc, e.g., 4 GB of size on a 32-bit host is something the DMA API under discussion would need to handle as error. To detect such an attempt, the API needs the guest's data type for the size. :) It would then either assert or return NULL.

Don't forget that this is not only about supposedly "dead" x86 hosts, imagine ppc64 on ppc or amd64 on arm etc.


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