Re: [PATCH 18/26] KVM: PPC: KVM PV guest stubs

Alexander Graf
2010-06-27T09:47:45Z

Am 27.06.2010 um 10:28 schrieb Avi Kivity <avi-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>:

> On 06/26/2010 02:25 AM, Alexander Graf wrote:
>> We will soon start and replace instructions from the text section
>> with
>> other, paravirtualized versions. To ease the readability of those
>> patches
>> I split out the generic looping and magic page mapping code out.
>>
>> This patch still only contains stubs. But at least it loops through
>> the
>> text section :).
>>
>>
>> +
>> +static void kvm_check_ins(u32 *inst)
>> +{
>> + u32 _inst = *inst;
>> + u32 inst_no_rt = _inst& ~KVM_MASK_RT;
>> + u32 inst_rt = _inst& KVM_MASK_RT;
>> +
>> + switch (inst_no_rt) {
>> + }
>> +
>> + switch (_inst) {
>> + }
>> +
>> + flush_icache_range((ulong)inst, (ulong)inst + 4);
>> +}
>>
>
> Shouldn't we flush only if we patched something?

We introduce the patching in the next patches. This is only a
preparation stub.

>
>> +
>> +static void kvm_use_magic_page(void)
>> +{
>> + u32 *p;
>> + u32 *start, *end;
>> +
>> + /* Tell the host to map the magic page to -4096 on all CPUs */
>> +
>> + on_each_cpu(kvm_map_magic_page, NULL, 1);
>> +
>> + /* Now loop through all code and find instructions */
>> +
>> + start = (void*)_stext;
>> + end = (void*)_etext;
>> +
>> + for (p = start; p< end; p++)
>> + kvm_check_ins(p);
>> +}
>> +
>>
>
> Or, flush the entire thing here.

I did that at first. It breaks. During the patching we may take
interrupts (pahe faults for example) that contain just patched
instructions. And really, hell breaks loose if we don't flush it
immediately :). I was hoping at first a 32 bit replace would be atomic
in cache, but the cpu tried to execute invalid instructions, so it
must have gotten some intermediate state.

Alex