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Re: Xen PAT settings vs Linux PAT settings
- To: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 14 Oct 2024 23:21:02 +0100
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- Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Mon, 14 Oct 2024 22:21:28 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 14/10/2024 10:37 pm, Marek Marczykowski-Górecki wrote:
> On Mon, Oct 14, 2024 at 09:05:58PM +0100, Andrew Cooper wrote:
>> On 14/10/2024 7:26 pm, Marek Marczykowski-Górecki wrote:
>>> Technically, it's a PV ABI violation, and it does break few things
>>> (definitely PV domU with passthrough are affected - Xen considers them
>>> L1TF vulnerable then; PV live migration is most likely broken too).
>> Do you have more information on this? The PAT bits shouldn't form any
>> part of L1TF considerations.
> https://github.com/QubesOS/qubes-issues/issues/8593
>
0x8010000018200066
That's a very L1TF-unsafe PTE, but it's also got nothing to do with PAT.
It's:
NX | Avail(bit 52) | addr (0x18200000) | D | A | U | W
and importantly not present. PAT == 0 == WB in both the Xen and Linux
worlds.
But, it likely does highlight a codepath which is opencoding PTE updates.
We really ought to have an option to do as f61c54967f4a did with
_PAGE_GNTTAB, and to inject #GP into the guest to get a backtrace out of
Linux.
In the case that we're going to crash the domain anyway, #GP is still
more useful, although I would quite like the #GP option instead of
shadowing too. Maybe hanging off pv-l1tf=fault as an option?
~Andrew
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