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

Marc Zyngier
2017-10-17T14:28:48Z

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.

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