From: Jan Beulich <jbeulich@suse.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
Kevin Tian <kevin.tian@intel.com>, Julien Grall <julien@xen.org>,
Andrew Cooper <andrew.cooper3@citrix.com>, Wei Liu <wl@xen.org>
Subject: Re: [PATCH] VMX: use a single, global APIC access page
Date: Thu, 11 Feb 2021 12:22:41 +0100 [thread overview]
Message-ID: <547b40f2-3b7b-10cb-30f6-9445c784eb0b@suse.com> (raw)
In-Reply-To: <YCUSDSYpS5X+AZco@Air-de-Roger>
On 11.02.2021 12:16, Roger Pau Monné wrote:
> On Thu, Feb 11, 2021 at 11:36:59AM +0100, Jan Beulich wrote:
>> On 11.02.2021 09:45, Roger Pau Monné wrote:
>>> On Wed, Feb 10, 2021 at 05:48:26PM +0100, Jan Beulich wrote:
>>>> --- a/xen/include/asm-x86/p2m.h
>>>> +++ b/xen/include/asm-x86/p2m.h
>>>> @@ -935,6 +935,9 @@ static inline unsigned int p2m_get_iommu
>>>> flags = IOMMUF_readable;
>>>> if ( !rangeset_contains_singleton(mmio_ro_ranges, mfn_x(mfn)) )
>>>> flags |= IOMMUF_writable;
>>>> + /* VMX'es APIC access page is global and hence has no owner. */
>>>> + if ( mfn_valid(mfn) && !page_get_owner(mfn_to_page(mfn)) )
>>>> + flags = 0;
>>>
>>> Is it fine to have this page accessible to devices if the page tables
>>> are shared between the CPU and the IOMMU?
>>
>> No, it's not, but what do you do? As said elsewhere, devices
>> gaining more access than is helpful is the price we pay for
>> being able to share page tables. But ...
>
> I'm concerned about allowing devices to write to this shared page, as
> could be used as an unintended way to exchange information between
> domains?
Well, such an abuse would be possible, but it wouldn't be part
of an ABI and hence could break at any time. Similarly I
wouldn't consider it an information leak if a guest abused
this.
>>> Is it possible for devices to write to it?
>>
>> ... thinking about it - they would be able to access it only
>> when interrupt remapping is off. Otherwise the entire range
>> 0xFEExxxxx gets treated differently altogether by the IOMMU,
>
> Now that I think of it, the range 0xFEExxxxx must always be special
> handled for device accesses, regardless of whether interrupt remapping
> is enabled or not, or else they won't be capable of delivering MSI
> messages?
I don't think I know how exactly chipsets handle MSI in this
case, but yes, presumably these accesses need to be routed a
different path even in that case.
> So I assume that whatever gets mapped in the IOMMU pages tables at
> 0xFEExxxxx simply gets ignored?
This would be the implication, aiui.
> Or else mapping the lapic access page when using shared page tables
> would have prevented CPU#0 from receiving MSI messages.
I guess I don't understand this part. In particular I see
nothing CPU#0 specific here.
Jan
next prev parent reply other threads:[~2021-02-11 11:23 UTC|newest]
Thread overview: 16+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-10 16:48 [PATCH] VMX: use a single, global APIC access page Jan Beulich
2021-02-10 17:00 ` Andrew Cooper
2021-02-10 17:03 ` Jan Beulich
2021-03-01 2:08 ` Tian, Kevin
2021-02-10 17:16 ` Jan Beulich
2021-02-11 8:45 ` Roger Pau Monné
2021-02-11 10:36 ` Jan Beulich
2021-02-11 11:16 ` Roger Pau Monné
2021-02-11 11:22 ` Jan Beulich [this message]
2021-02-11 12:27 ` Roger Pau Monné
2021-03-01 2:18 ` Tian, Kevin
2021-03-01 8:15 ` Jan Beulich
2021-03-01 8:30 ` Tian, Kevin
2021-03-01 9:58 ` Jan Beulich
2021-03-04 7:51 ` Tian, Kevin
2021-02-11 13:53 ` Andrew Cooper
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