Received-SPF: pass (google.com: domain of linux-kernel+bounces-205455-linux.lists.archive=gmail.com@vger.kernel.org designates 2604:1380:45d1:ec00::1 as permitted sender) client-ip=2604:1380:45d1:ec00::1;
Subject: Re: [PATCH] LoongArch: KVM: Add feature passing from user space
To: Huacai Chen <chenhuacai@kernel.org>
Cc: WANG Xuerui <kernel@xen0n.name>, Tianrui Zhao <zhaotianrui@loongson.cn>,
 kvm@vger.kernel.org, loongarch@lists.linux.dev, linux-kernel@vger.kernel.org
References: <20240606074850.2651896-1-maobibo@loongson.cn>
 <9bb552c8-fe86-43dc-9c4e-0b95c99fb25c@xen0n.name>
 <2774b010-8033-2167-474a-cb1b29b27d2b@loongson.cn>
 <ca286a23-f22b-092c-20d0-6ab20fd0883f@loongson.cn>
 <CAAhV-H4R7GO33jggAHsq0A6qLejdhnht5eBLs3OSasCkcYwJmQ@mail.gmail.com>
From: maobibo <maobibo@loongson.cn>
Message-ID: <fb5a5fb6-5c40-7d18-bb8d-2da3747480ff@loongson.cn>
Date: Fri, 7 Jun 2024 14:31:41 +0800
User-Agent: Mozilla/5.0 (X11; Linux loongarch64; rv:68.0) Gecko/20100101
 Thunderbird/68.7.0
Precedence: bulk
MIME-Version: 1.0
In-Reply-To: <CAAhV-H4R7GO33jggAHsq0A6qLejdhnht5eBLs3OSasCkcYwJmQ@mail.gmail.com>
Content-Type: text/plain; charset=utf-8; format=flowed
Content-Language: en-US
Content-Transfer-Encoding: 8bit


On 2024/6/7 上午11:58, Huacai Chen wrote:
> Hi, Bibo,
> 
> 
> On Thu, Jun 6, 2024 at 8:05 PM maobibo <maobibo@loongson.cn> wrote:
>>
>>
>>
>> On 2024/6/6 下午7:54, maobibo wrote:
>>> Xuerui,
>>>
>>> Thanks for your reviewing.
>>> I reply inline.
>>>
>>> On 2024/6/6 下午7:20, WANG Xuerui wrote:
>>>> Hi,
>>>>
>>>> On 6/6/24 15:48, Bibo Mao wrote:
>>>>> Currently features defined in cpucfg CPUCFG_KVM_FEATURE comes from
>>>>> kvm kernel mode only. Some features are defined in user space VMM,
>>>>
>>>> "come from kernel side only. But currently KVM is not aware of
>>>> user-space VMM features which makes it hard to employ optimizations
>>>> that are both (1) specific to the VM use case and (2) requiring
>>>> cooperation from user space."
>>> Will modify in next version.
>>>>
>>>>> however KVM module does not know. Here interface is added to update
>>>>> register CPUCFG_KVM_FEATURE from user space, only bit 24 - 31 is valid.
>>>>>
>>>>> Feature KVM_LOONGARCH_VCPU_FEAT_VIRT_EXTIOI is added from user mdoe.
>>>>> FEAT_VIRT_EXTIOI is virt EXTIOI extension which can route interrupt
>>>>> to 256 VCPUs rather than 4 CPUs like real hw.
>>>>
>>>> "A new feature bit ... is added which can be set from user space.
>>>> FEAT_... indicates that the VM EXTIOI can route interrupts to 256
>>>> vCPUs, rather than 4 like with real HW."
>>> will modify in next version.
>>>
>>>>
>>>> (Am I right in paraphrasing the "EXTIOI" part?)
>>>>
>>>>>
>>>>> Signed-off-by: Bibo Mao <maobibo@loongson.cn>
>>>>> ---
>>>>>    arch/loongarch/include/asm/kvm_host.h  |  4 +++
>>>>>    arch/loongarch/include/asm/loongarch.h |  5 ++++
>>>>>    arch/loongarch/include/uapi/asm/kvm.h  |  2 ++
>>>>>    arch/loongarch/kvm/exit.c              |  1 +
>>>>>    arch/loongarch/kvm/vcpu.c              | 36 +++++++++++++++++++++++---
>>>>>    5 files changed, 44 insertions(+), 4 deletions(-)
>>>>>
>>>>> diff --git a/arch/loongarch/include/asm/kvm_host.h
>>>>> b/arch/loongarch/include/asm/kvm_host.h
>>>>> index 88023ab59486..8fa50d757247 100644
>>>>> --- a/arch/loongarch/include/asm/kvm_host.h
>>>>> +++ b/arch/loongarch/include/asm/kvm_host.h
>>>>> @@ -135,6 +135,9 @@ enum emulation_result {
>>>>>    #define KVM_LARCH_HWCSR_USABLE    (0x1 << 4)
>>>>>    #define KVM_LARCH_LBT        (0x1 << 5)
>>>>> +#define KVM_LOONGARCH_USR_FEAT_MASK            \
>>>>> +    BIT(KVM_LOONGARCH_VCPU_FEAT_VIRT_EXTIOI)
>>>>> +
>>>>>    struct kvm_vcpu_arch {
>>>>>        /*
>>>>>         * Switch pointer-to-function type to unsigned long
>>>>> @@ -210,6 +213,7 @@ struct kvm_vcpu_arch {
>>>>>            u64 last_steal;
>>>>>            struct gfn_to_hva_cache cache;
>>>>>        } st;
>>>>> +    unsigned int usr_features;
>>>>>    };
>>>>>    static inline unsigned long readl_sw_gcsr(struct loongarch_csrs
>>>>> *csr, int reg)
>>>>> diff --git a/arch/loongarch/include/asm/loongarch.h
>>>>> b/arch/loongarch/include/asm/loongarch.h
>>>>> index 7a4633ef284b..4d9837512c19 100644
>>>>> --- a/arch/loongarch/include/asm/loongarch.h
>>>>> +++ b/arch/loongarch/include/asm/loongarch.h
>>>>> @@ -167,9 +167,14 @@
>>>>>    #define CPUCFG_KVM_SIG            (CPUCFG_KVM_BASE + 0)
>>>>>    #define  KVM_SIGNATURE            "KVM\0"
>>>>> +/*
>>>>> + * BIT 24 - 31 is features configurable by user space vmm
>>>>> + */
>>>>>    #define CPUCFG_KVM_FEATURE        (CPUCFG_KVM_BASE + 4)
>>>>>    #define  KVM_FEATURE_IPI        BIT(1)
>>>>>    #define  KVM_FEATURE_STEAL_TIME        BIT(2)
>>>>> +/* With VIRT_EXTIOI feature, interrupt can route to 256 VCPUs */
>>>>> +#define  KVM_FEATURE_VIRT_EXTIOI    BIT(24)
>>>>>    #ifndef __ASSEMBLY__
>>>>
>>>> What about assigning a new CPUCFG leaf ID for separating the two kinds
>>>> of feature flags very cleanly?
>>> For compatible issue like new kernel on old KVM host, to add a new
>>> CPUCFG leaf ID, a new feature need be defined on existing
>>> CPUCFG_KVM_FEATURE register. Such as:
>>>      #define  KVM_FEATURE_EXTEND_CPUCFG        BIT(3)
>>>
>>> VM need check feature KVM_FEATURE_EXTEND_CPUCFG at first, and then read
>>> the new CPUCFG leaf ID if feature EXTEND_CPUCFG is enabled.
>>>
>>> That maybe makes it complicated since feature bit is enough now.
>> The default return value is zero with old kvm host, it is possible to
>> use a new CPUCFG leaf ID. Both methods are ok for me.
>>
>> Huacai,
>> What is your optnion about this?
> I don't think we need a new leaf, but is this feature detection
> duplicated with EXTIOI_VIRT_FEATURES here?
> https://lore.kernel.org/lkml/871q5a2lo8.ffs@tglx/T/#t
It is for compatible consideration. The result is unknown on old 
hypervisor if new kernel accesses newly added IOCSR register 
EXTIOI_VIRT_FEATURES.

For cpucfg instruction it is returns zero if it is not supported, 
however there is no such spec for iocsr read or write instruction.

Regards
Bibo
> 
> Huacai
> 
>>
>> Regards
>> Bibo Mao
>>>>
>>>>> @@ -896,7 +907,24 @@ static int kvm_loongarch_vcpu_get_attr(struct
>>>>> kvm_vcpu *vcpu,
>>>>>    static int kvm_loongarch_cpucfg_set_attr(struct kvm_vcpu *vcpu,
>>>>>                         struct kvm_device_attr *attr)
>>>>>    {
>>>>> -    return -ENXIO;
>>>>> +    u64 __user *user = (u64 __user *)attr->addr;
>>>>> +    u64 val, valid_flags;
>>>>> +
>>>>> +    switch (attr->attr) {
>>>>> +    case CPUCFG_KVM_FEATURE:
>>>>> +        if (get_user(val, user))
>>>>> +            return -EFAULT;
>>>>> +
>>>>> +        valid_flags = KVM_LOONGARCH_USR_FEAT_MASK;
>>>>> +        if (val & ~valid_flags)
>>>>> +            return -EINVAL;
>>>>> +
>>>>> +        vcpu->arch.usr_features |= val;
>>>>
>>>> Isn't this usage of "|=" instead of "=" implying that the feature bits
>>>> could not get disabled after being enabled earlier, for whatever reason?
>>> yes, "=" is better. Will modify in next version.
>>>
>>> Regards
>>> Bibo Mao
>>>>
>>>>> +        return 0;
>>>>> +
>>>>> +    default:
>>>>> +        return -ENXIO;
>>>>> +    }
>>>>>    }
>>>>>    static int kvm_loongarch_vcpu_set_attr(struct kvm_vcpu *vcpu,
>>>>>
>>>>> base-commit: 2df0193e62cf887f373995fb8a91068562784adc
>>>>
>>>
>>
>>