From mboxrd@z Thu Jan  1 00:00:00 1970
From: Marc Zyngier <marc.zyngier@arm.com>
Subject: Re: [PATCH 23/27] arm64/sve: KVM: Hide SVE from CPU features exposed
 to guests
Date: Wed, 16 Aug 2017 12:10:53 +0100
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To: Dave Martin <Dave.Martin@arm.com>
Cc: linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org, libc-alpha@sourceware.org, Ard Biesheuvel <ard.biesheuvel@linaro.org>, Szabolcs Nagy <szabolcs.nagy@arm.com>, Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will.deacon@arm.com>, Richard Sandiford <richard.sandiford@arm.com>, kvmarm@lists.cs.columbia.edu, Christoffer Dall <christoffer.dall@linaro.org>

On 16/08/17 11:54, Dave Martin wrote:
> On Tue, Aug 15, 2017 at 05:37:55PM +0100, Marc Zyngier wrote:
>> On 09/08/17 13:05, 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>
>>> ---
>>>  arch/arm64/kvm/sys_regs.c | 14 +++++++++++++-
>>>  1 file changed, 13 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
>>> index 6583dd7..9e8c54e 100644
>>> --- a/arch/arm64/kvm/sys_regs.c
>>> +++ b/arch/arm64/kvm/sys_regs.c
>>> @@ -897,8 +897,20 @@ 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) {
>>> +		static bool printed;
>>> +
>>> +		if ((val & (0xfUL << ID_AA64PFR0_SVE_SHIFT)) && !printed) {
>>> +			kvm_info("SVE unsupported for guests, suppressing\n");
>>> +			printed = true;
>>> +		}
>>
>> Ideally, this should be a vcpu_unimpl_once(). But:
>> - it doesn't exist
>> - vcpu_unimpl looks hopelessly x86 specific
> 
> Yeah, I looked for an appropriate function and didn't find one ... and
> writing one just for this seemed overkill.
> 
>> How about turning it into a pr_err_once() instead?
> 
> Can do, though should it be an err?
> 
> No error has occurred here, rather I want people who discover that their
> guest mysteriously doesn't see SVE gets a clue about why.
An "err" is a good way to make it appear on the console. If you want to
make it noticed, that's the way.

	M.
-- 
Jazz is not dead. It just smells funny...