On Sat, 17 Mar 2012 09:59:23 +0100, Antonios Motakis <a.motakis at virtualopensystems.com> wrote: > Applies to commit 827ee5bffde826336c4a7f33109347c77a2cbbcf of the > latest kvm-a15-v7 branch. > > This patch allows to safely run guests that use the VFP and NEON > floating point extensions. When we switch to a guest, we save > the host's state (registers with floating point data, configuration > registers) and load those used by the guest. We can also handle > versions of the extensions with 16 or 32 double registers, and > also the registers introduced by the Common VFP Subarchitecture, as > defined in the ARM Architecture Reference Manual, Issue C. > > Possible future improvements include letting QEMU fetch a copy of the > registers, and lazy switching of the registers only when they are > actually used. I re-read this patch, and I don't think it's right: > +.macro store_vfp_state vcpu=0, vcpup > + mrc p10, 7, r2, cr8, cr0, 0 @ FPEXC > + tst r2, #FPEXC_EN @ Check if floating point is enabled > + beq 1f The guest may turn FPEXEC_EN off, but that doesn't mean the state isn't important. In particular, a (non-SMP) Linux ARM guest will turn the FPEXEC_EN bit off on context switch, without saving the contents. Similar logic applied to the host: we might have turned FPEXEC_EN off, but that doesn't mean the contents of the registers aren't worth saving. Cheers, Rusty.
