On 3 September 2011 12:26, Blue Swirl <blauwir...@gmail.com> wrote: > On Fri, Sep 2, 2011 at 6:08 AM, Sinha, Ani <ani.si...@tellabs.com> 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
