Re: [edk2-devel] [PATCH] UefiCpuPkg: Fix issue that IsModified is wrongly set in PageTableMap

["Laszlo Ersek" <lersek@redhat.com>]

On 1/15/24 03:59, Liu, Zhiguang wrote:
> Hi Laszlo,
> 
> I don't think it is a good idea to explicitly mask out the Accessed/Dirty bit. We can't assume Accessed/Dirty bit are only changed by hardware, because the caller also can change the Accessed/Dirty bit.
> 
> For API PageTableMap, the IsModified is already set as False in the beginning of the function.
> For internal function PageTableLibMapInLevel, we don't set IsModified as False in the beginning on purpose, because it keeps the global state of whether the PageTable is changed.
> 
> I plan to change the comment as below to explicitly explain the behavior:
> 
> For internal function PageTableLibMapInLevel, the description of IsModified should be:
> @param[in, out]     IsModified     change IsModified to True if page table is modified and input parameter Modify is TRUE.
> 
> For API PageTableMap, the description of IsModified should be: 
> @param[out]     IsModified     TRUE means page table is modified by software or hardware. FALSE means page table is not modified by software.
> If the output IsModified is FALSE, there is possibility that the page table is changed by hardware. It is ok because page table can be changed by hardware anytime, and we don't need to Flush TLB.
> 
> With these comments changed, I don't need to change C code in my patch.

OK, sounds reasonable to me. Thanks.

> 
> BTW, I assume IsModified can be used as an indicator whether the caller need to flush TLB. Do you prefer to change the parameter name to IsFlushTlbNeeded? I am both fine.

I think the new description of the parameter suffices (without renaming
the parameter).

Laszlo



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