Re: [Qemu-devel] general question

[Peter Maydell]

On 31 May 2012 10:21, 陳韋任 (Wei-Ren Chen) <address@hidden> wrote:
> void tlb_flush_page(CPUArchState *env, target_ulong addr)
> {
>    if ((addr & env->tlb_flush_mask) == env->tlb_flush_addr) {
>        tlb_flush(env, 1); --- (1)
>        return;
>    }

This case (see the comments) is only for handling "large pages".
tlb_flush_addr/tlb_flush_mask track the range covered by large
pages, so that if we attempt to flush a page for an address in
that region we do a full tlb flush instead. (cf tlb_add_large_page()).
However the normal case is for standard sized pages:

>    ... snip ...
>
>    addr &= TARGET_PAGE_MASK;
>    i = (addr >> TARGET_PAGE_BITS) & (CPU_TLB_SIZE - 1);
>    for (mmu_idx = 0; mmu_idx < NB_MMU_MODES; mmu_idx++) {
>        tlb_flush_entry(&env->tlb_table[mmu_idx][i], addr);
>    }
>
>    tb_flush_jmp_cache(env, addr);

...which as you see is only flushing the tlb/tb entries
corresponding to the specified page address.

>  The comment of tlb_flush (1) says,
>
>    QEMU doesn't currently implement a global/not-global flag
>    for tlb entries, at the moment tlb_flush() will also flush all
>    tlb entries in the flush_global == false case.

This is something different. Most CPUs support some sort of concept
of TLB entries which are marked as specific to a particular OS
process, so that the OS can avoid having to do a full TLB flush
when it does a context switch. The "global" terminology is a
reference to the x86 specific flavour of this, and the QEMU API
currently reflects that x86ism. However we don't actually implement
the global/non-global distinction, hence this comment (it's always
safe to flush more entries than you're asked to flush).

-- PMM