Re: [PATCH 1/1] KVM: use KVM_HVA_ERR_BAD to check bad hva

From: "Huang, Kai" <kai.huang@intel.com>
To: Dongli Zhang <dongli.zhang@oracle.com>, <kvm@vger.kernel.org>
Cc: <pbonzini@redhat.com>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/1] KVM: use KVM_HVA_ERR_BAD to check bad hva
Date: Fri, 1 Mar 2024 10:53:22 +1300	[thread overview]
Message-ID: <04398f4e-6098-4559-9604-b9810753801e@intel.com> (raw)
In-Reply-To: <20240229212522.34515-1-dongli.zhang@oracle.com>

On 1/03/2024 10:25 am, Dongli Zhang wrote:
> Replace PAGE_OFFSET with KVM_HVA_ERR_BAD, to facilitate the cscope when
> looking for where KVM_HVA_ERR_BAD is used.
> 
> Every time I use cscope to query the functions that are impacted by the
> return value (KVM_HVA_ERR_BAD) of __gfn_to_hva_many(), I may miss
> kvm_is_error_hva().

I am not sure "to facilitate cscope" could be a justification to do some 
code change in the kernel.

> 
> Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
> ---
>   include/linux/kvm_host.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 7e7fd25b09b3..4dc0300e7766 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -143,7 +143,7 @@ static inline bool is_noslot_pfn(kvm_pfn_t pfn)
>   
>   static inline bool kvm_is_error_hva(unsigned long addr)
>   {
> -	return addr >= PAGE_OFFSET;
> +	return addr >= KVM_HVA_ERR_BAD;
>   }
>   
>   #endif

Also, IIUC the KVM_HVA_ERR_BAD _theoretically_ can be any random value 
that can make kvm_is_error_hva() return false, while kvm_is_error_hva() 
must catch all error HVAs.

E.g., if we ever change KVM_HVA_ERR_BAD to use any other value (although 
I don't see why this could ever happen), then using KVM_HVA_ERR_BAD in 
kvm_is_error_hva() would be broken.

In other words, it seems to me we should just use PAGE_OFFSET in 
kvm_is_error_hva().