[Qemu-devel] Address translation - virt->phys->ram

[Ian Molton <ian.molton@collabora.co.uk>]

Hi folks,

I've been updating some old patches which make use of a function to
translate guest virtual addresses into pointers into the guest RAM.

As I understand it qemu has guest virtual and physical addresses, the
latter of which map somehow to host ram addresses.

The function which the code had been using appears not to work under
kvm, which leads me to think that qemu doesnt emulate the MMU (or at
least not in the same manner) when it is using kvm as opposed to pure
emulation.

If I turn off kvm, the patch works, albeit slowly. If I enable it, the
code takes the path which looks for the magic value (below).

Is there a 'proper' way to translate guest virtual addresses into host
RAM addresses?

Here is the code:-

static /*inline*/ void *get_phys_mem_addr(CPUState *env, target_ulong addr)
{
    int mmu_idx;
    int index;
        int i;

    index = (addr >> TARGET_PAGE_BITS) & (CPU_TLB_SIZE - 1);
    mmu_idx = cpu_mmu_index(env);

    if (__builtin_expect
        (env->tlb_table[mmu_idx][index].addr_code !=
         (addr & TARGET_PAGE_MASK), 0)) {
        target_ulong ret = cpu_get_phys_page_debug((CPUState *) env, addr);
        if (ret == -1) {
            fprintf(stderr,
                    "not in phys mem " TARGET_FMT_lx "(" TARGET_FMT_lx " "
                    TARGET_FMT_lx ")\n", addr,
                    env->tlb_table[mmu_idx][index].addr_code,
                    addr & TARGET_PAGE_MASK);
            fprintf(stderr, "cpu_x86_handle_mmu_fault = %d\n",
                    cpu_x86_handle_mmu_fault((CPUState *) env, addr, 0,
mmu_idx, 1));
            return NULL;
        } else {
            if (ret + TARGET_PAGE_SIZE <= ram_size) {
                return qemu_get_ram_ptr((ret + (((target_ulong) addr) &
(TARGET_PAGE_SIZE - 1))));
            } else {
                fprintf(stderr,
                        "cpu_get_phys_page_debug(env, " TARGET_FMT_lx ")
== "
                        TARGET_FMT_lx "\n", addr, ret);
                fprintf(stderr,
                        "ram_size= " TARGET_FMT_lx "\n", ret,
(target_ulong) ram_size);

        for(i = 0 ; i < ram_size-10 ; i++) {
                char *ptr = qemu_get_ram_ptr(i);
                if(!strncmp("magic_string", ptr, 10)) {
                        fprintf(stderr, "found magic_string at: %lx
%lx\n", i, ptr);
                        break;
                }
        }

                return qemu_get_ram_ptr(i-128); //Evil horrible hack
            }
        }
    } else
        return (void *) addr + env->tlb_table[mmu_idx][index].addend;
}