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

[Dave Martin <Dave dot Martin at arm dot com>]

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?

Cheers
---Dave