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Re: [Qemu-devel] target_phys_addr_t vs ram_addr_t

From: Sinha, Ani
Subject: Re: [Qemu-devel] target_phys_addr_t vs ram_addr_t
Date: Wed, 7 Sep 2011 14:25:40 -0500

Thanks guys for the explanations. I'd try and add some comments to the code 
explaining the apis once I have a better understanding of the code. Right now I 
am still digesting the undocumented apis.


On Sep 3, 2011, at 6:14 AM, Peter Maydell wrote:

> On 3 September 2011 12:26, Blue Swirl <address@hidden> wrote:
>> On Fri, Sep 2, 2011 at 6:08 AM, Sinha, Ani <address@hidden> wrote:
>>> First off, what is the difference between target_phys_addr_t and
>>> ram_addr_t? I believe the former is a virtual address within the guest but
>>> what is the later? The comment says "address in ram (different from
>>> physical address)" but is this the virtual address or the physical
>>> address? Is this for guest or for host?
>> target_phys_addr_t is used for guest physical addresses. For example,
>> i386 guest with PAE would need 36 bits which makes this 64 bit type
>> (regardless of host address sizes). Devices should usually only use
>> this type.
>> ram_addr_t (uintptr_t) is linked to the size of host virtual address
>> space. Because of how RAM is implemented, we can't give 64 bit guests
>> more than 4 GB of RAM on a 32 bit host, so in that case it would be 32
>> bits. On a 64 bit host this can be 64 bits. It can be considered as a
>> subset of target_phys_addr_t. Normally devices need it only if there
>> are RAM areas, like frame buffers.
> To add to this and point out some particular wrinkles:
> Even if on the guest machine RAM doesn't start at physical address
> 0, the first bit of RAM will generally be at a zero ram_addr_t.
> If the guest machine has some RAM that is mapped at two physical
> addresses, then both those target_phys_addr_t values will map to
> the same ram_addr_t. This is why you can't just cast a ram_addr_t
> to a target_phys_addr_t or vice-versa. (This kind of situation
> doesn't happen on the PC but does on some of the embedded boards
> qemu models.)
> I think of ram_addr_t as being "offset into a big lump of host
> memory which we have parcelled out to use as guest RAM".
> -- PMM

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