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Re: [Xen-devel] Xen 4.14 and future work
- To: Xen-devel List <xen-devel@xxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Thu, 5 Dec 2019 15:30:58 +0000
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On 02/12/2019 19:51, Andrew Cooper wrote:
> Hello,
>
> Now that 4.13 is on its way out of the door, it is time to look to
> ongoing work.
(and some more...)
* Shim: Removal of an M2P.
Within the shim, two M2P's are constructed, and one of them is entirely
unused. Both take up a decent chunk of memory, an contribute to reduced
packing density
By inspecting the kernel width earlier during boot, we can avoid
creating the unneeded M2P. This would require teaching Xen to operate
with only a compat p2m when running a 32bit guest, but this will be fine
to run with.
* CONFIG_PV32
Despite being deprecated in 64bit processors, we still use ring 1 for
32bit PV kernels. A relic of a bygone era, this causes problems with
newer pagetable based features (SMEP and SMAP in particular), resulting
in complexity in the entry paths and a substantial performance penalty
(a CR4 write on the way in and out of 32bit guests). There are also
some speculative mitigation actions we take unilaterally, just because
32bit PV guest kernels run on supervisor mappings rather than user
mappings (RSB-stuffing being the most obvious one).
From an attack surface point of view, being able to remove all ring 1
facilities would be great for deployments not intending to run 32bit PV
guests (and/or relegate them to PV-shim), and there will be some
(probably minor) performance gains for 64bit PV guests as a consequence
of the simplified entry paths.
(And who knows... A combination of this and Paul's idea for emulation
thunk-ing in a pseudo SMM-like mode inside HVM guests could remove the
need for CONFIG_COMPAT entirely at L0.)
* CONFIG_PDX
Here is one I prepared earlier.
https://andrewcoop-xen.readthedocs.io/en/docs-devel/misc/tech-debt.html#config-pdx
* CONFIG_$VENDOR
For restricted usecases, dropping one of AMD or INTEL would drop a
substantial chunk of code, and a sufficiently capable LTO compiler with
devirtualisation support could even remove the function pointers themselves.
More usefully to the general project however, would be RANDCONFIG's
ability to check and keep our vendor-specific interfaces clean. They
most definitely are not right now.
~Andrew
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