Re: [Xen-devel] paging mechanism clarification

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

>  The most important difference is that in PV mode the pagetables the guest
>  generates are directly used by the host processor. For this reason, there
>  are various changes required to the way the guest updates its pagetables.Â
>  The most important of these is that the guest must translate its
>  pseudophysical addresses to host machine addresses before filling out page
>  table entries.
>
>  Thanks for the clarification Mark.
>  But Isn't Xen hypevisor supposed to do this translation of pseuophysical
> addresses to host machine address, instead of the guest itself?

Not for paravirtualised (Xen-aware) guests.  They handle their own 
translations, which makes it possible to eliminate shadow paging entirely for 
them; this is a benefit to performance.  Shadow pagetables are normally only 
used for a PV guest when live migration is in progress.

HTH,
Cheers,
Mark

>  Thank you
>  --pradeep
>
>  In PV mode, pages that are currently part of a pagetable are only ever
> allowed to be mapped readonly in order to prevent tampering by the guest.
>
>  Cheers,
>  Mark
>
>  > > > I hope i made myself clear.
>  > > > Please enlighten me :-).
>  > > >
>  > > > When paging is enabled, we use a shadow page-table, which is
>  > > > essentially
>  > > > that the GUEST sees one page-table, and the processor another
>  > > > (thanks to
>  > > > the fact that the hypervisor intercepts the CR3 read/write
>  > >
>  > > operations,
>  > >
>  > > > and when CR3 is read back by the guest, we don't send back the value
>  > > > it's ACTUALLY POINTING TO IN THE PROCESSOR, but the value
>  > >
>  > > that was set
>  > >
>  > > > by the guest). So there are two page-tables.
>  > > >
>  > > > Got this well, thanks Mats :).
>  > > >
>  > > > To make the page-table updates by the guest visible to the
>  > >
>  > > hypervisor,
>  > >
>  > > > all of the guest-page-tables are made read-only (by scanning
>  > > > the new CR3
>  > > > value whenever one is set).
>  > > >
>  > > > I didn't get this either well :(
>  > > > sorry, but do you mean CR3 for the guest or for the
>  > > > processor? i hope you mean guest?
>  > >
>  > > Yes, scan the guest-CR3 to see where it placed the page-tables.
>  > >
>  > > > Whenever a page-fault happens, the hypervisor has "first look", and
>  > > > determines if the update is for a page-table or not. If it is a
>  > > > page-table update, the guest operation is emulated (in
>  > >
>  > > x86_emulate.c),
>  > >
>  > > > and the result is written to the shadow-page-table AND the
>  > > >
>  > > > Why do we need emulation?some peculiar reason for emulating?
>  > > > Do you mean to say if i am running a 32 bit domU on top of a
>  > > > 64 bit processor, the guest operation for updating the page
>  > > > table is emulated by the hypervisor.am i right?
>  > >
>  > > No, it's simply because we need to see the result of the
>  > > instruction and
>  > > write it to two places (with some modification in one of
>  > > those places).
>  > > So if the code is doing, for example: "*pte |= 1;" (set a
>  > > page-table-entry to "present"), we need to mark both the
>  > > guest-page-table-entry to "present", and mark our
>  > > shadow-entry "present"
>  > > (and perhaps do some other work too, but that's the minimum work
>  > > needed).
>  > >
>  > > This brings one more question in my mind.Why do we use pinning then?
>  >
>  > I believe there's two types of pinning! Page-pinning, which is blocking
>  > a page from being accessed in an incorrect way [again, I'm not 100% sure
>  > how this works, or exactly what it does - just that it's a term used in
>  > the general way I described in the previous sentence].
>  >
>  > > As i see at it.To avoid shadow page tables to be swapped out
>  > > before the page tables they actually point to are swapped.Am i right?
>  > >
>  > > But according to interface manual,-> to bind a vcpu to a
>  > > specific CPU in a SMP environment we use pining.But these two
>  > > look pretty orthogonal statements to me, which means i may be
>  > > wrong :(.
>  > > Can somebody help me in this regard?
>  >
>  > CPU pinning is to tie a VCPU to a (set of) processor(s). For example,
>  > you may want to pin Dom0 to run only on CPU0, and pin a DomU to run on
>  > CPU's 1,2 and 3. That way, Dom0 is ALWAYS able to run on it's own CPU,
>  > and it's never in contention about which CPU to use, and DomU can run on
>  > three CPU's as much as it likes. You could have another DomU pinned to
>  > CPU 3 if you wish. That means that CPU 1, 2 are exclusively for the
>  > first DomU, whilst the second DomU shares CPU3 with the first DomU (so
>  > they both get half the CPU performance of one CPU - on average over a
>  > reasonable amount of time).
>  >
>  > --
>  > Mats
>  >
>  > > Pointers to actual code will be of great help.
>  > >
>  > > Thanks a lot Mats.
>  > > Thank you all.
>  > >
>  > > --pradeep
>  > >
>  > > > Does this means on a x86 platform this overkill or this
>  > > > emulation is skipped altogether?
>  > > > Please bear with me as i am an absolute Xen newbie out here :-).
>  > >
>  > > No, it's ALWAYS used for all page-table writes, as far as I
>  > > understand.
>  > >
>  > > --
>  > > Mats
>  > >
>  > > > guest-page-table, but in the shadow-page-table, the value is
>  > > > modified to
>  > > > reflect the actual address in machine-space, rather than what
>  > > > the guest
>  > > > thinks it should be.
>  > > >
>  > > > In futuer versions of AMD processors (and I believe Intel are
>  > > > working on
>  > > > something very similar if not the same), there will be a mode
>  > > > where the
>  > > > processor is able to work in "nested paging mode", which means that
>  > > > there are two "parallel" page-tables. The first one is the
>  > > > "guest-page-table", the second one is the "host-page-table". In this
>  > > > case, every lookup in the guest-page-table will be done through the
>  > > > host-page-table. So we have a "simple" way to just take the
>  > > > guest-page-table and translate it to machine-physical-address
>  > > > - with the
>  > > > good thing that the host-page-table needn't change, since the
>  > > > pages that
>  > > > the host consists of is pretty much static for the duration of the
>  > > > guest.
>  > > >
>  > > > Yes, read about about this in an article mention how Pacifica
>  > > > is better than VT.
>  > > >
>  > > > Say for example, we have a guest that lives at 256-512MB. The
>  > > > guest-page-table would contain, for example, a mapping for
>  > > > 0x12200000 ->
>  > > > guest-physical 0x100000 (1MB). The host-page-table
>  > >
>  > > translates this to
>  > >
>  > > > 0x10100000 because the 1MB entry in guest-address is 256+1MB in
>  > > > machine-address.
>  > > >
>  > > > Exactly, got this well on spot :).
>  > > >
>  > > > [In reality, it's very likely that the guest never gets all
>  > > > the space in
>  > > > one big chunk, but rather a few pages here and a few pages there. If
>  > > > there are big chunks, we could use large pages to map those!].
>  > > >
>  > > > Thanks a ton Mats and all.
>  > > >
>  > > > --pradeep
>  > > >
>  > > > The support for nested paging (called HAP, Hardware Assisted
>  > > > Paging) is
>  > > > in the Unstable version of Xen since a few days back.
>  > > >
>  > > > --
>  > > > Mats
>  > > >
>  > > > > And this whole 2 level paging consitutes Xen's shadow page
>  > > > > tables. Right?
>  > > > >
>  > > > > Is my understanding of Xen's paging mechanism correct?or am i
>  > > > > missing something?
>  > > > >
>  > > > > Thank you
>  > > > >
>  > > > > -pradeep
>  >
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>
>  --
>  Dave: Just a question. What use is a unicyle with no seat? And no pedals!
>  Mark: To answer a question with a question: What use is a skateboard?
>  Dave: Skateboards have wheels.
>  Mark: My wheel has a wheel!

-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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