Re: [PATCH 4/4] arm/arm64: KVM: use kernel mapping to perform invalidation on page fault

[Christoffer Dall <christoffer.dall@linaro.org>]

On Thu, Jan 08, 2015 at 03:21:50PM +0000, Peter Maydell wrote:
> On 8 January 2015 at 15:06, Marc Zyngier <marc.zyngier@arm.com> wrote:
> > On 08/01/15 13:16, Peter Maydell wrote:
> >>> ASID cached VIVT icaches are also VMID tagged. It is thus impossible for
> >>> stale cache lines to come with a new page. And if by synchronizing the
> >>> caches you obtain a different instruction stream, it means you've
> >>> restored the wrong page.
> >>
> >> ...is that true even if the dirty data in the dcache comes from
> >> the userspace process doing DMA or writing the initial boot
> >> image or whatever?
> >
> > We perform this on a page that is being brought in stage-2. Two cases:
> >
> > - This is a page is mapped for the first time: the icache should be
> > invalid for this page (the guest should have invalidated it the first
> > place),
> 
> If this is the first instruction in the guest (ie we've just
> (warm) reset the VM and are running the kernel as loaded into the guest
> by QEMU/kvmtool) then the guest can't have invalidated the icache,
> and QEMU can't do the invalidate because it doesn't have the vaddr
> and VMID of the guest.
> 
The guest must clean its icache before turning on the MMU, no?

Whenever we reuse a VMID (rollover), we flush the entire icache for that
vmid.

-Christoffer