linux-kernel - Re: [PATCH v2 23/49] KVM: x86: Handle kernel- and KVM-defined CPUID words in a single helper

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Message-ID: <bd0006c9-2386-4d81-9147-f262262d70ce@intel.com>
Date: Wed, 17 Jul 2024 22:00:31 +0800
From: Xiaoyao Li <xiaoyao.li@...el.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Maxim Levitsky <mlevitsk@...hat.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
 Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org,
 linux-kernel@...r.kernel.org, Hou Wenlong <houwenlong.hwl@...group.com>,
 Kechen Lu <kechenl@...dia.com>, Oliver Upton <oliver.upton@...ux.dev>,
 Binbin Wu <binbin.wu@...ux.intel.com>,
 Yang Weijiang <weijiang.yang@...el.com>,
 Robert Hoo <robert.hoo.linux@...il.com>
Subject: Re: [PATCH v2 23/49] KVM: x86: Handle kernel- and KVM-defined CPUID
 words in a single helper

On 7/9/2024 5:18 AM, Sean Christopherson wrote:
> On Thu, Jul 04, 2024, Maxim Levitsky wrote:
>> On Fri, 2024-05-17 at 10:39 -0700, Sean Christopherson wrote:
>>> Merge kvm_cpu_cap_init() and kvm_cpu_cap_init_kvm_defined() into a single
>>> helper.  The only advantage of separating the two was to make it somewhat
>>> obvious that KVM directly initializes the KVM-defined words, whereas using
>>> a common helper will allow for hardening both kernel- and KVM-defined
>>> CPUID words without needing copy+paste.
>>>
>>> No functional change intended.
>>>
>>> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@...el.com>

>>> ---
>>>   arch/x86/kvm/cpuid.c | 44 +++++++++++++++-----------------------------
>>>   1 file changed, 15 insertions(+), 29 deletions(-)
>>>
>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>>> index f2bd2f5c4ea3..8efffd48cdf1 100644
>>> --- a/arch/x86/kvm/cpuid.c
>>> +++ b/arch/x86/kvm/cpuid.c
>>> @@ -622,37 +622,23 @@ static __always_inline u32 raw_cpuid_get(struct cpuid_reg cpuid)
>>>   	return *__cpuid_entry_get_reg(&entry, cpuid.reg);
>>>   }
>>>   
>>> -/* Mask kvm_cpu_caps for @leaf with the raw CPUID capabilities of this CPU. */
>>> -static __always_inline void __kvm_cpu_cap_mask(unsigned int leaf)
>>> +static __always_inline void kvm_cpu_cap_init(u32 leaf, u32 mask)
>>>   {
>>>   	const struct cpuid_reg cpuid = x86_feature_cpuid(leaf * 32);
>>>   
>>> -	reverse_cpuid_check(leaf);
>>> +	/*
>>> +	 * For kernel-defined leafs, mask the boot CPU's pre-populated value.
>>> +	 * For KVM-defined leafs, explicitly set the leaf, as KVM is the one
>>> +	 * and only authority.
>>> +	 */
>>> +	if (leaf < NCAPINTS)
>>> +		kvm_cpu_caps[leaf] &= mask;
>>> +	else
>>> +		kvm_cpu_caps[leaf] = mask;
>>
>> Hi,
>>
>> I have an idea, how about we just initialize the kvm only leafs to 0xFFFFFFFF
>> and then treat them exactly in the same way as kernel regular leafs?
>>
>> Then the user won't have to figure out (assuming that the user doesn't read
>> the comment, who does?) why we use mask as init value.
>>
>> But if you prefer to leave it this way, I won't object either.
> 
> Huh, hadn't thought of that.  It's a small code change, but I'm leaning towards
> keeping the current code as we'd still need a comment to explain why KVM sets
> all bits by default.  

> And in the unlikely case that we royally screw up and fail
> to call kvm_cpu_cap_init() on a word, starting with 0xff would result in all
> features in the uninitialized word being treated as supported.

+1

> For posterity...
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index 18ded0e682f2..6fcfb0fa4bd6 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -762,11 +762,7 @@ do {                                                                       \
>          u32 kvm_cpu_cap_emulated = 0;                                   \
>          u32 kvm_cpu_cap_synthesized = 0;                                \
>                                                                          \
> -       if (leaf < NCAPINTS)                                            \
> -               kvm_cpu_caps[leaf] &= (mask);                           \
> -       else                                                            \
> -               kvm_cpu_caps[leaf] = (mask);                            \
> -                                                                       \
> +       kvm_cpu_caps[leaf] &= (mask);                                   \
>          kvm_cpu_caps[leaf] &= (raw_cpuid_get(cpuid) |                   \
>                                 kvm_cpu_cap_synthesized);                \
>          kvm_cpu_caps[leaf] |= kvm_cpu_cap_emulated;                     \
> @@ -780,7 +776,7 @@ do {                                                                        \
>   
>   void kvm_set_cpu_caps(void)
>   {
> -       memset(kvm_cpu_caps, 0, sizeof(kvm_cpu_caps));
> +       memset(kvm_cpu_caps, 0xff, sizeof(kvm_cpu_caps));
>   
>          BUILD_BUG_ON(sizeof(kvm_cpu_caps) - (NKVMCAPINTS * sizeof(*kvm_cpu_caps)) >
>                       sizeof(boot_cpu_data.x86_capability));
>