Re: [PATCH v3 24/28] arm64/sve: KVM: Hide SVE from CPU features exposed to guests

[Christoffer Dall <cdall@linaro.org>]

On Tue, Oct 17, 2017 at 04:47:08PM +0100, Dave Martin wrote:
> On Tue, Oct 17, 2017 at 03:29:36PM +0100, Marc Zyngier wrote:
> > On 17/10/17 15:07, Dave Martin wrote:
> > > On Tue, Oct 17, 2017 at 06:58:16AM -0700, Christoffer Dall wrote:
> > >> On Tue, Oct 10, 2017 at 07:38:41PM +0100, Dave Martin wrote:
> > >>> KVM guests cannot currently use SVE, because SVE is always
> > >>> configured to trap to EL2.
> > >>>
> > >>> However, a guest that sees SVE reported as present in
> > >>> ID_AA64PFR0_EL1 may legitimately expect that SVE works and try to
> > >>> use it.  Instead of working, the guest will receive an injected
> > >>> undef exception, which may cause the guest to oops or go into a
> > >>> spin.
> > >>>
> > >>> To avoid misleading the guest into believing that SVE will work,
> > >>> this patch masks out the SVE field from ID_AA64PFR0_EL1 when a
> > >>> guest attempts to read this register.  No support is explicitly
> > >>> added for ID_AA64ZFR0_EL1 either, so that is still emulated as
> > >>> reading as zero, which is consistent with SVE not being
> > >>> implemented.
> > >>>
> > >>> This is a temporary measure, and will be removed in a later series
> > >>> when full KVM support for SVE is implemented.
> > >>>
> > >>> Signed-off-by: Dave Martin <Dave.Martin@arm.com>
> > >>> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
> > >>> Cc: Marc Zyngier <marc.zyngier@arm.com>
> > >>> ---
> > >>>  arch/arm64/kvm/sys_regs.c | 12 +++++++++++-
> > >>>  1 file changed, 11 insertions(+), 1 deletion(-)
> > >>>
> > >>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> > >>> index b1f7552..a0ee9b0 100644
> > >>> --- a/arch/arm64/kvm/sys_regs.c
> > >>> +++ b/arch/arm64/kvm/sys_regs.c
> > >>> @@ -23,6 +23,7 @@
> > >>>  #include <linux/bsearch.h>
> > >>>  #include <linux/kvm_host.h>
> > >>>  #include <linux/mm.h>
> > >>> +#include <linux/printk.h>
> > >>>  #include <linux/uaccess.h>
> > >>>  
> > >>>  #include <asm/cacheflush.h>
> > >>> @@ -897,8 +898,17 @@ static u64 read_id_reg(struct sys_reg_desc const *r, bool raz)
> > >>>  {
> > >>>  	u32 id = sys_reg((u32)r->Op0, (u32)r->Op1,
> > >>>  			 (u32)r->CRn, (u32)r->CRm, (u32)r->Op2);
> > >>> +	u64 val = raz ? 0 : read_sanitised_ftr_reg(id);
> > >>>  
> > >>> -	return raz ? 0 : read_sanitised_ftr_reg(id);
> > >>> +	if (id == SYS_ID_AA64PFR0_EL1) {
> > >>> +		if (val & (0xfUL << ID_AA64PFR0_SVE_SHIFT))
> > >>> +			pr_err_once("kvm [%i]: SVE unsupported for guests, suppressing\n",
> > >>> +				    task_pid_nr(current));
> > >>
> > >> nit: does this really qualify as an error print?
> > > 
> > > I have no strong opinion on this: maz suggested I should add this --
> > > his concern was to make it difficult to ignore.
> > > 
> > > This is transitional: the main purpose is to circumvent bug reports from
> > > people who find that SVE doesn't work in their guests, in the interim
> > > before proper KVM support lands upstream.
> > > 
> > > Marc, do you still agree with this position?
> > 
> > As long as this is transitional, I'm OK with this.
> 
> No argument from me, since it was your request in the first place ;)
> 
> Christoffer?
> 
No (further) argument from me.

Thanks,
-Christoffer