[PATCH] RISC-V: KVM: provide UAPI for host SATP mode

[Daniel Henrique Barboza]

On 7/5/23 09:18, Alexandre Ghiti wrote:
> 
> On 05/07/2023 11:15, Daniel Henrique Barboza wrote:
>> KVM userspaces need to be aware of the host SATP to allow them to
>> advertise it back to the guest OS.
>>
>> Since this information is used to build the guest FDT we can't wait for
> 
> 
> The thing is the "mmu-type" property in the FDT is never used: the kernel will probe the hardware and choose the largest available mode, or use "no4lvl"/"no5lvl" from the command line to restrict this mode. And FYI the current mode is exposed through cpuinfo. @Conor Can we deprecate this node or something similar?
> 
> Just a remark, not sure that helps :)

It does, thanks. I am aware that the current mode is exposed through cpuinfo.
mvendorid/marchid/mimpid is also exposed there. As far as I understand we should
rely on KVM to provide all CPU related info to configure a vcpu though.

A little background of where I'm coming from. One of the QEMU KVM cpu types (host)
doesn't have an assigned satp_mode. The FDT creation of the 'virt' board relies on
that info being present, and the result is that the board will segfault. I sent a
fix for it that I hope will be queued shortly:

https://lore.kernel.org/qemu-devel/20230630100811.287315-3-dbarboza@ventanamicro.com/

Thus, if it's decided that the satp_mode FDT is deprecated, we can ignore this
patch altogether. Thanks,


Daniel


> 
> 
>> the SATP reg to be readable. We just need to read the SATP mode, thus
>> we can use the existing 'satp_mode' global that represents the SATP reg
>> with MODE set and both ASID and PPN cleared. E.g. for a 32 bit host
>> running with sv32 satp_mode is 0x80000000, for a 64 bit host running
>> sv57 satp_mode is 0xa000000000000000, and so on.
>>
>> Add a new userspace virtual config register 'satp_mode' to allow
>> userspace to read the current SATP mode the host is using with
>> GET_ONE_REG API before spinning the vcpu.
>>
>> 'satp_mode' can't be changed via KVM, so SET_ONE_REG is allowed as long
>> as userspace writes the existing 'satp_mode'.
>>
>> Signed-off-by: Daniel Henrique Barboza <dbarboza at ventanamicro.com>
>> ---
>>   arch/riscv/include/asm/csr.h      | 2 ++
>>   arch/riscv/include/uapi/asm/kvm.h | 1 +
>>   arch/riscv/kvm/vcpu.c             | 7 +++++++
>>   3 files changed, 10 insertions(+)
>>
>> diff --git a/arch/riscv/include/asm/csr.h b/arch/riscv/include/asm/csr.h
>> index b6acb7ed115f..be6e5c305e5b 100644
>> --- a/arch/riscv/include/asm/csr.h
>> +++ b/arch/riscv/include/asm/csr.h
>> @@ -46,6 +46,7 @@
>>   #ifndef CONFIG_64BIT
>>   #define SATP_PPN    _AC(0x003FFFFF, UL)
>>   #define SATP_MODE_32    _AC(0x80000000, UL)
>> +#define SATP_MODE_SHIFT    31
>>   #define SATP_ASID_BITS    9
>>   #define SATP_ASID_SHIFT    22
>>   #define SATP_ASID_MASK    _AC(0x1FF, UL)
>> @@ -54,6 +55,7 @@
>>   #define SATP_MODE_39    _AC(0x8000000000000000, UL)
>>   #define SATP_MODE_48    _AC(0x9000000000000000, UL)
>>   #define SATP_MODE_57    _AC(0xa000000000000000, UL)
>> +#define SATP_MODE_SHIFT    60
>>   #define SATP_ASID_BITS    16
>>   #define SATP_ASID_SHIFT    44
>>   #define SATP_ASID_MASK    _AC(0xFFFF, UL)
>> diff --git a/arch/riscv/include/uapi/asm/kvm.h b/arch/riscv/include/uapi/asm/kvm.h
>> index f92790c9481a..0493c078e64e 100644
>> --- a/arch/riscv/include/uapi/asm/kvm.h
>> +++ b/arch/riscv/include/uapi/asm/kvm.h
>> @@ -54,6 +54,7 @@ struct kvm_riscv_config {
>>       unsigned long marchid;
>>       unsigned long mimpid;
>>       unsigned long zicboz_block_size;
>> +    unsigned long satp_mode;
>>   };
>>   /* CORE registers for KVM_GET_ONE_REG and KVM_SET_ONE_REG */
>> diff --git a/arch/riscv/kvm/vcpu.c b/arch/riscv/kvm/vcpu.c
>> index 8bd9f2a8a0b9..b31acf923802 100644
>> --- a/arch/riscv/kvm/vcpu.c
>> +++ b/arch/riscv/kvm/vcpu.c
>> @@ -313,6 +313,9 @@ static int kvm_riscv_vcpu_get_reg_config(struct kvm_vcpu *vcpu,
>>       case KVM_REG_RISCV_CONFIG_REG(mimpid):
>>           reg_val = vcpu->arch.mimpid;
>>           break;
>> +    case KVM_REG_RISCV_CONFIG_REG(satp_mode):
>> +        reg_val = satp_mode >> SATP_MODE_SHIFT;
>> +        break;
>>       default:
>>           return -EINVAL;
>>       }
>> @@ -395,6 +398,10 @@ static int kvm_riscv_vcpu_set_reg_config(struct kvm_vcpu *vcpu,
>>           else
>>               return -EBUSY;
>>           break;
>> +    case KVM_REG_RISCV_CONFIG_REG(satp_mode):
>> +        if (reg_val != (satp_mode >> SATP_MODE_SHIFT))
>> +            return -EINVAL;
>> +        break;
>>       default:
>>           return -EINVAL;
>>       }