[PATCH v2 1/3] KVM: arm64: Allow setting of ID_AA64PFR0_EL1.CSV2 from userspace

[Will Deacon]

On Tue, Nov 10, 2020 at 02:13:06PM +0000, Marc Zyngier wrote:
> We now expose ID_AA64PFR0_EL1.CSV2=1 to guests running on hosts
> that are immune to Spectre-v2, but that don't have this field set,
> most likely because they predate the specification.
> 
> However, this prevents the migration of guests that have started on
> a host the doesn't fake this CSV2 setting to one that does, as KVM
> rejects the write to ID_AA64PFR0_EL2 on the grounds that it isn't
> what is already there.
> 
> In order to fix this, allow userspace to set this field as long as
> this doesn't result in a promising more than what is already there
> (setting CSV2 to 0 is acceptable, but setting it to 1 when it is
> already set to 0 isn't).
> 
> Fixes: e1026237f9067 ("KVM: arm64: Set CSV2 for guests on hardware unaffected by Spectre-v2")
> Reported-by: Peng Liang <liangpeng10 at huawei.com>
> Signed-off-by: Marc Zyngier <maz at kernel.org>
> ---
>  arch/arm64/include/asm/kvm_host.h |  2 ++
>  arch/arm64/kvm/arm.c              | 16 ++++++++++++
>  arch/arm64/kvm/sys_regs.c         | 42 ++++++++++++++++++++++++++++---
>  3 files changed, 56 insertions(+), 4 deletions(-)

Acked-by: Will Deacon <will at kernel.org>

Will