[PATCH v4 00/12] arm/arm64: KVM: host cache maintenance when guest caches are off
catalin.marinas at arm.com
Wed Feb 19 05:12:46 EST 2014
On Wed, Feb 19, 2014 at 09:02:34AM +0000, Marc Zyngier wrote:
> On 2014-02-18 20:57, Eric Northup wrote:
> > On Tue, Feb 18, 2014 at 7:27 AM, Marc Zyngier <marc.zyngier at arm.com>
> > wrote:
> >> When we run a guest with cache disabled, we don't flush the cache to
> >> the Point of Coherency, hence possibly missing bits of data that
> >> have
> >> been written in the cache, but have not yet reached memory.
> >> We also have the opposite issue: when a guest enables its cache,
> >> whatever sits in the cache is suddenly going to become visible,
> >> shadowing whatever the guest has written into RAM.
> >> There are several approaches to these issues:
> >> - Using the DC bit when caches are off: this breaks guests assuming
> >> caches off while doing DMA operations. Bootloaders, for example.
> >> It also breaks the I-D coherency.
> >> - Fetch the memory attributes on translation fault, and flush the
> >> cache while handling the fault. This relies on using the PAR_EL1
> >> register to obtain the Stage-1 memory attributes, and tends to be
> >> slow.
> >> - Detecting the translation faults occuring with MMU off (and
> >> performing a cache clean), and trapping SCTLR_EL1 to detect the
> >> moment when the guest is turning its caches on (and performing a
> >> cache invalidation). Trapping of SCTLR_EL1 is then disabled to
> >> ensure the best performance.
> > This will preclude noticing the 2nd .. Nth cache off -> on cycles,
> > right? Will any guests care - doesn't kexec go through a caches-off
> > state?
> kexec, bootloaders, and whatever firmware requires to switch caches on
> and then off. Guest does care, but we don't have an (efficient)
> architectural solution to that.
> The best I can think of so far is a "switch-the-damned-thing-off"
> hypercall that would do the above before returning to the guest.
We could push for a PSCI extension to cover such cases as well, though
even for a host, we may not need to involve the secure world for kexec.
An alternative is to trap the set/way cache flushing and re-activate the
MMU trapping in the guest. If the MMU is still on, disable trapping
until the next set/way (since that's a normal function on power-down
code sequences). But it doesn't look nice ;).
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