[Differential] D9686: fix UEFI VM's bootup on Hyper-V (i.e. Hyper-V Generation-2 VM)

[decui_microsoft.com (Dexuan Cui)]

decui_microsoft.com added inline comments.

INLINE COMMENTS

> kib wrote in copy.c:79-84
> Note that at the loader/earliest kernel boot phase, we must consider two different types of addresses, physical and virtual.  Kernel is linked at specific virtual address, and for the text segment of the kernel, it does not matter a bit which physical addresses back the virtual mappings, as far as the virtual mapping satisfies the expectation of linker.  In principle, the physical pages do not need to be contiguous even.
> 
> The physical backing is, of course, important too, but it affects different things.  In particular, vm_page_array[] pages must be correctly removed from the runs, for the physical addresses that are used to map kernel.  Another place which should be adapted is the creation of the initial kernel page tables, in create_pagetables() pmap function.  The function makes explicit assumption that the kernel is mapped by contig physical pages.
> 
> Another question is how the kernel comprehends the used physical pages to map it.  Kernel could try to inspect the initial (loader) page table it is started with, but it needs some way to access memory by a physical addresses for that.  Or loader might pass some additional table to inform kernel about initial mapping, to avoid the need to inspect the page table.

IIRC, the UEFI firmware enables protected mode with identity-mapped paging (i.e. VA==PA) , before the execution is transferred to FreeBSD's EFI loader. So here IMO I can treat VA==PA in the patch (it looks FreeBSD UEFI loader doesn't change the VA/PA mapping(?)).

I have to admit the fact that I don't know the loader very well and actually it's my first time to have the chance to read some of the code and try to make a patch for it. :-)

Please kindly elaborate a little more about the issues that you think exist in the patch and I am happy to dig more into them.

REVISION DETAIL
  https://reviews.freebsd.org/D9686

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To: decui_microsoft.com, imp, jhb, will, kib, delphij, emaste, sepherosa_gmail.com, honzhan_microsoft.com, howard0su_gmail.com, marcel
Cc: freebsd-virtualization-list