Re: [Xen-devel] [PATCH v10 5/6] x86/ioreq server: Asynchronously reset outstanding p2m_ioreq_server entries.

[Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>]

On 4/6/2017 1:01 AM, George Dunlap wrote:
> On 05/04/17 17:32, Yu Zhang wrote:
> > On 4/6/2017 12:35 AM, George Dunlap wrote:
> > > On 05/04/17 17:22, Yu Zhang wrote:
> > > > On 4/5/2017 10:41 PM, George Dunlap wrote:
> > > > > On Sun, Apr 2, 2017 at 1:24 PM, Yu Zhang <yu.c.zhang@xxxxxxxxxxxxxxx>
> > > > > wrote:
> > > > > > After an ioreq server has unmapped, the remaining p2m_ioreq_server
> > > > > > entries need to be reset back to p2m_ram_rw. This patch does this
> > > > > > asynchronously with the current p2m_change_entry_type_global()
> > > > > > interface.
> > > > > >
> > > > > > New field entry_count is introduced in struct p2m_domain, to record
> > > > > > the number of p2m_ioreq_server p2m page table entries. One nature of
> > > > > > these entries is that they only point to 4K sized page frames, because
> > > > > > all p2m_ioreq_server entries are originated from p2m_ram_rw ones in
> > > > > > p2m_change_type_one(). We do not need to worry about the counting for
> > > > > > 2M/1G sized pages.
> > > > > Assuming that all p2m_ioreq_server entries are *created* by
> > > > > p2m_change_type_one() may valid, but can you assume that they are only
> > > > > ever *removed* by p2m_change_type_one() (or recalculation)?
> > > > >
> > > > > What happens, for instance, if a guest balloons out one of the ram
> > > > > pages?  I don't immediately see anything preventing a p2m_ioreq_server
> > > > > page from being ballooned out, nor anything on the
> > > > > decrease_reservation() path decreasing p2m->ioreq.entry_count.  Or did
> > > > > I miss something?
> > > > >
> > > > > Other than that, only one minor comment...
> > > > Thanks for your thorough consideration, George. But I do not think we
> > > > need to worry about this:
> > > >
> > > > If the emulation is in process, the balloon driver cannot get a
> > > > p2m_ioreq_server page - because
> > > > it is already allocated.
> > > In theory, yes, the guest *shouldn't* do this.  But what if the guest OS
> > > makes a mistake?  Or, what if the ioreq server makes a mistake and
> > > places a watch on a page that *isn't* allocated by the device driver, or
> > > forgets to change a page type back to ram when the device driver frees
> > > it back to the guest kernel?
> > Then the lazy p2m change code will be triggered, and this page is reset
> > to p2m_ram_rw
> > before being set to p2m_invalid, just like the normal path. Will this be
> > a problem?
> No, I'm talking about before the ioreq server detaches.
Sorry, I do not get it. Take scenario 1 for example:
> Scenario 1: Bug in driver
> 1. Guest driver allocates page A
> 2. dm marks A as p2m_ioreq_server
Here in step 2. the ioreq.entry_count increases;
> 3. Guest driver accidentally frees A to the kernel
> 4. guest kernel balloons out page A; ioreq.entry_count is wrong
Here in step 4. the ioreq.entry_count decreases.
Isn't this what we are expecting?

Yu
> Scenario 2: Bug in the kernel
> 1. Guest driver allocates page A
> 2. dm marks A as p2m_ioreq_server
> 3. Guest kernel tries to balloon out page B, but makes a calculation
> mistake and balloons out A instead; now ioreq.entry_count is wrong
>
> Scenario 3: Off-by-one bug in devicemodel
> 1. Guest driver allocates pages A-D
> 2. dm makes a mistake and marks pages A-E as p2m_ioreq_server (one extra
> page)
> 3. guest kernel balloons out page E; now ioreq.entry_count is wrong
>
> Scenario 4: "Leak" in devicemodel
> 1. Guest driver allocates page A
> 2. dm marks A as p2m_ioreq_server
> 3. Guest driver is done with page A, but DM forgets to reset it to
> p2m_ram_rw
> 4. Guest driver frees A to guest kernel
> 5. Guest kernel balloons out page A; now ioreq.entry_count is wrong
>
> I could keep going on; there are *lots* of bugs in the driver, the
> kernel, or the devicemodel which could cause pages marked
> p2m_ioreq_server to end up being ballooned out; which under the current
> code would make ioreq.entry_count wrong.
>
> It's the hypervisor's job to do the right thing even when other
> components have bugs in them.  This is why I initially suggested keeping
> count in atomic_write_ept_entry() -- no matter how the entry is changed,
> we always know exactly how many entries of type p2m_ioreq_server we have.
>
>   -George

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