Re: [RFC v2 0/7] kvm: arm64: emulate ID registers

[Peng Liang <liangpeng10@huawei.com>]

On 9/18/2020 4:01 PM, Andrew Jones wrote:
> On Thu, Sep 17, 2020 at 08:00:54PM +0800, Peng Liang wrote:
>> In AArch64, guest will read the same values of the ID regsiters with
>> host.  Both of them read the values from arm64_ftr_regs.  This patch
>> series add support to emulate and configure ID registers so that we can
>> control the value of ID registers that guest read.
>>
>> v1 -> v2:
>>  - save the ID registers in sysreg file instead of a new struct
>>  - apply a checker before setting the value to the register
>>  - add doc for new KVM_CAP_ARM_CPU_FEATURE
>>
>> Peng Liang (7):
>>   arm64: add a helper function to traverse arm64_ftr_regs
>>   arm64: introduce check_features
>>   kvm: arm64: save ID registers to sys_regs file
>>   kvm: arm64: introduce check_user
>>   kvm: arm64: implement check_user for ID registers
>>   kvm: arm64: make ID registers configurable
>>   kvm: arm64: add KVM_CAP_ARM_CPU_FEATURE extension
>>
>>  Documentation/virt/kvm/api.rst      |   8 +
>>  arch/arm64/include/asm/cpufeature.h |   4 +
>>  arch/arm64/include/asm/kvm_coproc.h |   2 +
>>  arch/arm64/include/asm/kvm_host.h   |   3 +
>>  arch/arm64/kernel/cpufeature.c      |  36 +++
>>  arch/arm64/kvm/arm.c                |   3 +
>>  arch/arm64/kvm/sys_regs.c           | 481 +++++++++++++++++++++++++++-
>>  arch/arm64/kvm/sys_regs.h           |   6 +
>>  include/uapi/linux/kvm.h            |   1 +
>>  9 files changed, 532 insertions(+), 12 deletions(-)
>>
>> -- 
>> 2.26.2
>>
> 
> Hi Peng,
> 
> I'd much rather see a series of patches where each patch converts a single
> ID register from using ID_SANITISED() to having its own table entry, where
> its own set_user() and reset() functions take into account its features
> using high level arm64_ftr* functions. Any ID registers that can still
> share code can certainly do so with some post-conversion refactoring.
> 
> Thanks,
> drew
> 
> .
> 
Hi Andrew,

Thank you for your advise.  I'll rework the code to use different set_user()
for different ID registers (maybe some general registers still use shareing
set_user()) and check the value defined by user space in its own set_user in
next version.

But do we need to implement reset() for ID registers?  I think ID registers
are read-only in guest and guest won't and can't change their values.  And
after 03fdfb2690099 ("KVM: arm64: Don't write junk to sysregs on reset"),
we won't write junk to sysregs on reset.  So their values won't change on
reset?

Thanks,
Peng
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