[RFC/RFT PATCH 0/3] arm64: KVM: work around incoherency with uncached guest mappings

[Andrew Jones]

On Tue, Mar 03, 2015 at 07:13:48PM +0100, Laszlo Ersek wrote:
> On 03/03/15 18:34, Alexander Graf wrote:
> > On 02/19/2015 11:54 AM, Ard Biesheuvel wrote:
> >> This is a 0th order approximation of how we could potentially force
> >> the guest
> >> to avoid uncached mappings, at least from the moment the MMU is on.
> >> (Before
> >> that, all of memory is implicitly classified as Device-nGnRnE)
> >>
> >> The idea (patch #2) is to trap writes to MAIR_EL1, and replace
> >> uncached mappings
> >> with cached ones. This way, there is no need to mangle any guest page
> >> tables.
> >>
> >> The downside is that, to do this correctly, we need to always trap
> >> writes to
> >> the VM sysreg group, which includes registers that the guest may write
> >> to very
> >> often. To reduce the associated performance hit, patch #1 introduces a
> >> fast path
> >> for EL2 to perform trivial sysreg writes on behalf of the guest,
> >> without the
> >> need for a full world switch to the host and back.
> >>
> >> The main purpose of these patches is to quantify the performance hit, and
> >> verify whether the MAIR_EL1 handling works correctly.
> > 
> > I gave this a quick spin on a VM running with QEMU.
> > 
> >   * VGA output is still distorted, I get random junk black lines in the
> > output in between
> >   * When I add -device nec-usb-xhci -device usb-kbd the VM doesn't even
> > boot up
> 
> Do you also have the dirty page tracking patches in your host kernel? I
> needed both (and got them via Drew's backport, thanks) and then both VGA
> and USB started working fine.

Assuming you have the dirty page tracking already, then you're probably
missing the fixup to patch 2/3, s/0xbb/0xff/

> 
> Without the MAIR patches, I got cache-line size "random" corruptions in
> the VGA display (16 pixel wide small segments). Without dirty page
> tracking, big chunks (sometimes even almost the entire screen) was blank.
> 
> Regarding USB, unless you have both of the patchsets in the host kernel,
> the guest will indeed crash early during boot. Gerd confirmed for me
> that "usb controller (all uhci/ehci/xhci) pci regions see both read
> (status bits) and write (control bits) access". So if there's any
> corruption in there, on read, that looks like a malfunctioning piece of
> hw for the guest kernel, and in this case it happens to crash.
> 
> > With TCG, both bits work fine.
> 
> Yep.
> 
> Thanks
> Laszlo
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