On 2/29/24 13:53, Huang, Kai wrote:
>
>
> 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@xxxxxxxxxx>
>> ---
>> 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().
>
At least so far PAGE_OFFSET is the same value as KVM_HVA_ERR_BAD (except
mips/s390), as line 141. Therefore, this is "No functional change".
It indicates the userspace VMM can never have hva in the range of kernel space.
139 #ifndef KVM_HVA_ERR_BAD
140
141 #define KVM_HVA_ERR_BAD (PAGE_OFFSET)
142 #define KVM_HVA_ERR_RO_BAD (PAGE_OFFSET + PAGE_SIZE)
143
144 static inline bool kvm_is_error_hva(unsigned long addr)
145 {
146 return addr >= PAGE_OFFSET;
147 }
148
149 #endif
Regarding to "facilitate cscope", this happened since long time ago when I read
about ept_violation/mmio path.
1. The __gfn_to_hva_many() may return KVM_HVA_ERR_BAD for mmio.
2. Then I used cscope to find the location of KVM_HVA_ERR_BAD.
3. The kvm_is_error_hva() is not in the results.
4. It took me a while to figure out that the 'KVM_HVA_ERR_BAD' is indirectly
used by kvm_is_error_hva().
This is just based on my own experience when reading mmio code path. Thank you
very much!
Dongli Zhang
