From: Jan Beulich <jbeulich@suse.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: Andrew Cooper <andrew.cooper3@citrix.com>, Wei Liu <wl@xen.org>,
George Dunlap <george.dunlap@citrix.com>,
Julien Grall <julien@xen.org>,
Stefano Stabellini <sstabellini@kernel.org>,
xen-devel@lists.xenproject.org
Subject: Re: [PATCH] x86/ept: limit calls to memory_type_changed()
Date: Tue, 27 Sep 2022 08:35:20 +0200 [thread overview]
Message-ID: <82fb70ca-25cf-6e06-968d-416798f65f79@suse.com> (raw)
In-Reply-To: <YzHMDIxjXQ+txtwy@MacBook-Air-de-Roger.local>
On 26.09.2022 17:58, Roger Pau Monné wrote:
> On Mon, Sep 26, 2022 at 05:36:41PM +0200, Jan Beulich wrote:
>> On 26.09.2022 17:25, Roger Pau Monné wrote:
>>> Correction: the Arm memory_type_changed() needs to stay, as
>>> iomem_{permit,deny}_access() is common code.
>>
>> Right, or we'd need some other arch abstraction. (I wonder whether
>> long term Arm can actually get away without this. Even on the AMD side
>> of x86 I don't think it's quite right that adding/removing of MMIO
>> ranges has no effect on the memory type of accesses.)
>
> IIRC there's no way for the hypervisor to infer cache attributes on
> AMD SVM for NPT entries, but maybe I'm missing something. Guest MTRRs
> settings are completely ignored for AMD guests.
Right, as documented: "Note that there is no hardware support for guest
MTRRs; the VMM can simulate their effect by altering the memory types
in the nested page tables." That's something we imo should do, but which
I don't think we actually do (see p2m_type_to_flags()). We respect the
PAT bit when splitting large pages, but I don't think we ever set the
bit when making new / updated entries.
> I'm not able ATM
> however to find in the AMD PM how effective cache attributes are
> calculated when using NPT however. I would guess host MTRR + guest
> PAT?
First guest and host PAT are combined, then the result is combined with
(host) MTRR. See the tables in the "Nested Paging" sub-section "Combining
Memory Types, MTRRs". Of course things are quite a bit more limited (but
also simpler) in shadow mode.
Jan
next prev parent reply other threads:[~2022-09-27 6:35 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-22 16:05 [PATCH] x86/ept: limit calls to memory_type_changed() Roger Pau Monne
2022-09-22 19:21 ` Jan Beulich
2022-09-23 8:35 ` Roger Pau Monné
2022-09-26 7:33 ` Jan Beulich
2022-09-26 14:50 ` Roger Pau Monné
2022-09-26 15:25 ` Roger Pau Monné
2022-09-26 15:36 ` Jan Beulich
2022-09-26 15:58 ` Roger Pau Monné
2022-09-27 6:35 ` Jan Beulich [this message]
2022-09-27 8:40 ` Roger Pau Monné
2022-09-26 18:03 ` Andrew Cooper
2022-09-27 9:33 ` Roger Pau Monné
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