Re: [Xen-devel] [PATCH 0/2] MMIO emulation fixes

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.08.18 at 17:35, <Paul.Durrant@xxxxxxxxxx> wrote:
>>  -----Original Message-----
>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: 10 August 2018 16:31
>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
>> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; xen-devel <xen-
>> devel@xxxxxxxxxxxxxxxxxxxx>
>> Subject: RE: [Xen-devel] [PATCH 0/2] MMIO emulation fixes
>> 
>> >>> On 10.08.18 at 17:08, <Paul.Durrant@xxxxxxxxxx> wrote:
>> >>  -----Original Message-----
>> >> From: Andrew Cooper
>> >> Sent: 10 August 2018 13:56
>> >> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; 'Jan Beulich'
>> >> <JBeulich@xxxxxxxx>
>> >> Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
>> >> Subject: Re: [Xen-devel] [PATCH 0/2] MMIO emulation fixes
>> >>
>> >> On 10/08/18 13:43, Paul Durrant wrote:
>> >> >> -----Original Message-----
>> >> >> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> >> Sent: 10 August 2018 13:37
>> >> >> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
>> >> >> Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
>> >> >> Subject: RE: [Xen-devel] [PATCH 0/2] MMIO emulation fixes
>> >> >>
>> >> >>>>> On 10.08.18 at 14:22, <Paul.Durrant@xxxxxxxxxx> wrote:
>> >> >>>>  -----Original Message-----
>> >> >>>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> >>>> Sent: 10 August 2018 13:13
>> >> >>>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
>> >> >>>> Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
>> >> >>>> Subject: RE: [Xen-devel] [PATCH 0/2] MMIO emulation fixes
>> >> >>>>
>> >> >>>>>>> On 10.08.18 at 14:08, <Paul.Durrant@xxxxxxxxxx> wrote:
>> >> >>>>>>  -----Original Message-----
>> >> >>>>>> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> >> >>>>>> Sent: 10 August 2018 13:02
>> >> >>>>>> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>
>> >> >>>>>> Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
>> >> >>>>>> Subject: Re: [Xen-devel] [PATCH 0/2] MMIO emulation fixes
>> >> >>>>>>
>> >> >>>>>>>>> On 10.08.18 at 12:37, <paul.durrant@xxxxxxxxxx> wrote:
>> >> >>>>>>> These are probably both candidates for back-port.
>> >> >>>>>>>
>> >> >>>>>>> Paul Durrant (2):
>> >> >>>>>>>   x86/hvm/ioreq: MMIO range checking completely ignores
>> >> direction
>> >> >> flag
>> >> >>>>>>>   x86/hvm/emulate: make sure rep I/O emulation does not cross
>> >> GFN
>> >> >>>>>>>     boundaries
>> >> >>>>>>>
>> >> >>>>>>>  xen/arch/x86/hvm/emulate.c | 17 ++++++++++++++++-
>> >> >>>>>>>  xen/arch/x86/hvm/ioreq.c   | 15 ++++++++++-----
>> >> >>>>>>>  2 files changed, 26 insertions(+), 6 deletions(-)
>> >> >>>>>> I take it this isn't yet what we've talked about yesterday on irc?
>> >> >>>>>>
>> >> >>>>> This is the band-aid fix. I can now show correct handling of a rep
>> mov
>> >> >>>>> walking off MMIO into RAM.
>> >> >>>> But that's not the problem we're having. In our case the bad
>> behavior
>> >> >>>> is with a single MOV. That's why I had assumed that your plan to
>> fiddle
>> >> >>>> with null_handler would help in our case as well, while this series
>> >> clearly
>> >> >>>> won't (afaict).
>> >> >>>>
>> >> >>> Oh, I see. A single MOV spanning MMIO and RAM has undefined
>> >> behaviour
>> >> >> though
>> >> >>> as I understand it. Am I incorrect?
>> >> >> I'm not aware of SDM or PM saying anything like this. Anyway, the
>> >> >> specific case where this is being observed as an issue is when
>> >> >> accessing the last few bytes of a normal RAM page followed by a
>> >> >> ballooned out one. The balloon driver doesn't remove the virtual
>> >> >> mapping of such pages (presumably in order to not shatter super
>> >> >> pages); observation is with the old XenoLinux one, but from code
>> >> >> inspection the upstream one behaves the same.
>> >> >>
>> >> >> Unless we want to change the balloon driver's behavior, at least
>> >> >> this specific case needs to be considered having defined behavior,
>> >> >> I think.
>> >> >>
>> >> > Ok. I'll see what I can do.
>> >>
>> >> It is a software error to try and cross boundaries.  Modern processors
>> >> do their best to try and cause the correct behaviour to occur, albeit
>> >> with a massive disclaimer about the performance hit.  Older processors
>> >> didn't cope.
>> >>
>> >> As far as I'm concerned, its fine to terminate a emulation which crosses
>> >> a boundary with the null ops.
>> >
>> > Alas we never even get as far as the I/O handlers in some circumstances...
>> >
>> > I just set up a variant of an XTF test doing a backwards rep movsd into a
>> > well aligned stack buffer where source buffer starts 1 byte before a
>> boundary
>> > between RAM and MMIO. The code in hvmemul_rep_movs() (rightly)
>> detects that
>> > both the source and dest of the initial rep are RAM, skips over the I/O
>> > emulation calls, and then fails when the hvm_copy_from_guest_phys()
>> > (unsurprisingly) fails to grab the 8 bytes for the initial rep.
>> > So, any logic we add to deal with handling page spanning ops is going to
>> > have to go in at the top level of instruction emulation... which I fear is
>> > going to be quite a major change and not something that's going to be easy
>> to
>> > back-port.
>> 
>> Well, wasn't it clear from the beginning that a proper fix would be too
>> invasive to backport? And wasn't it for that reason that you intended
>> to add a small hack first, to deal with just the case(s) that we currently
>> have issues with?
> 
> Well, given that I mistakenly understood you were hitting the same rep issue 
> that I was, I thought I could deal with it in a reasonably straightforward 
> way. Maybe I can still do a point fix for what you are hitting though. What 
> precisely are you hitting? Always a single MOV? And always from a page 
> spanning source to a well aligned dest? Or more combinations than that?

Always a single, misaligned MOV spanning the boundary from a valid
RAM page to a ballooned out one (Linux'es load_unaligned_zeropad()).
I meanwhile wonder though whether it might not be better to address
this at the p2m level, by inserting a r/o mapping of an all-zeros page.
George, do you have any opinion one way or the other?

Jan


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