Re: [PATCH RFC 0/4] 5-level EPT

[Yu Zhang <yu.c.zhang@linux.intel.com>]

On 3/9/2017 10:16 PM, Paolo Bonzini wrote:
>
> On 17/01/2017 03:18, Li, Liang Z wrote:
>>> On 29/12/2016 10:25, Liang Li wrote:
>>>> x86-64 is currently limited physical address width to 46 bits, which
>>>> can support 64 TiB of memory. Some vendors require to support more for
>>>> some use case. Intel plans to extend the physical address width to
>>>> 52 bits in some of the future products.
>>>>
>>>> The current EPT implementation only supports 4 level page table, which
>>>> can support maximum 48 bits physical address width, so it's needed to
>>>> extend the EPT to 5 level to support 52 bits physical address width.
>>>>
>>>> This patchset has been tested in the SIMICS environment for 5 level
>>>> paging guest, which was patched with Kirill's patchset for enabling
>>>> 5 level page table, with both the EPT and shadow page support. I just
>>>> covered the booting process, the guest can boot successfully.
>>>>
>>>> Some parts of this patchset can be improved. Any comments on the
>>>> design or the patches would be appreciated.
>>> I will review the patches.  They seem fairly straightforward.
>>>
>>> However, I am worried about the design of the 5-level page table feature
>>> with respect to migration.
>>>
>>> Processors that support the new LA57 mode can write 57-canonical/48-
>>> noncanonical linear addresses to some registers even when LA57 mode is
>>> inactive.  This is true even of unprivileged instructions, in particular
>>> WRFSBASE/WRGSBASE.
>>>
>>> This is fairly bad because, if a guest performs such a write (because of a bug
>>> or because of malice), it will not be possible to migrate the virtual machine to
>>> a machine that lacks LA57 mode.
>>>
>>> Ordinarily, hypervisors trap CPUID to hide features that are only present in
>>> some processors of a heterogeneous cluster, and the hypervisor also traps
>>> for example CR4 writes to prevent enabling features that were masked away.
>>> In this case, however, the only way for the hypervisor to prevent the write
>>> would be to run the guest with
>>> CR4.FSGSBASE=0 and trap all executions of WRFSBASE/WRGSBASE.  This
>>> might have negative effects on performance for workloads that use the
>>> instructions.
>>>
>>> Of course, this is a problem even without your patches.  However, I think it
>>> should be addressed first.  I am seriously thinking of blacklisting FSGSBASE
>>> completely on LA57 machines until the above is fixed in hardware.
>>>
>>> Paolo
>> The issue has already been forwarded to the hardware guys, still waiting for the feedback.
> Going to review this now.  Any news?

Thanks for your reivew, Paolo.
This is Yu Zhang from Intel. I'll pick up this 5 level ept feature, and 
will try to address your comments next. :-)
Now I am learning Liang's code and trying to bring VM up with Kirill's 
native 5 level paging code integrated.

Yu
> Paolo
>