Re: [Qemu-devel] [PATCH] target-i386: Sanity check host processor physical address width

[Laszlo Ersek <lersek@redhat.com>]

On 07/09/15 21:11, Bandan Das wrote:
> Laszlo Ersek <lersek@redhat.com> writes:
> ...
>>>>
>>>> First, see my comments on the KVM patch.
>>>>
>>>> Second, ram_size is not the right thing to compare. What should be
>>>> checked is whether the highest guest-physical address that maps to RAM
>>>> can be represented in the address width of the host processor (and only
>>>> if EPT is enabled, but that sub-condition belongs to the KVM patch).
>>>>
>>>> Note that this is not the same as the check written in the patch. For
>>>> example, if you assume a 32-bit PCI hole with size 1 GB, then a total
>>>> guest RAM of size 63 GB will result in the highest guest-phys memory
>>>> address being 0xF_FFFF_FFFF, which just fits into 36 bits.
>>>>
>>>> Correspondingly, the above code would not print the warning for
>>>>
>>>>   -m $((63 * 1024 + 1))
>>>>
>>>> on my laptop (which has "address sizes   : 36 bits physical, ..."), even
>>>> though such a guest would not boot for me (with EPT enabled).
>>>>
>>>> Please see
>>>>
>>>> http://thread.gmane.org/gmane.comp.bios.tianocore.devel/15418/focus=15447
>>>>
>>>> So, "ram_size" in the controlling expression should be replaced with
>>>> "maximum_guest_ram_address" (which should be inclusive, and the <= relop
>>>> should be preserved).
>>> also with memory hotplug tuned on we should check if the end of
>>> hotplug memory area is less then limit, i.e.:
>>>
>>>   pcms->hotplug_memory.base + hotplug_mem_size < 1ULL << max_phys_bits
>>
>> Seems reasonable, thanks for the hint!
> 
> Thanks Igor and Laszlo, makes sense. I am wondering if this 1GB PCI
> hole is always fixed so that I can simply include that in calculating the maximum
> guest ram address ? Or do we have to figure that out every time ?

Please grep the tree for "above_4g_mem_size". The size of the 32-bit PCI
hole is not constant, but all the necessary computation goes into
"above_4g_mem_size" already.

So I think you should derive the max possible gpa from
"above_4g_mem_size" and the top of the hotpluggable memory area, and
compare that against the PCPU address width, *if* EPT is enabled.

(BTW "pcms->hotplug_memory.base" depends on "above_4g_mem_size" too.)

Thanks
Laszlo

> 
>> (The LHS in this instance is exclusive though, so equality should *not*
>> trigger the warning. "maximum_guest_ram_address" is inclusive, and
>> equality should trigger the warning. (Although equality seems quite
>> impossible in practice.))
>>
>> Thanks!
>> Laszlo