On 18 May 2018 at 02:50, Prakhya, Sai Praneeth <sai.praneeth.prakhya@xxxxxxxxx> wrote: > + Ricardo > >> > > I recently noticed in UEFI spec v2.7, section 2.3.4, Calling >> > > Conventions for X64, that “All selectors set to be flat with virtual >> > > = physical address. If the UEFI OS loader or OS used >> > > SetVirtualAddressMap() to relocate the runtime services in a virtual >> > > address space, then this condition does not have to be met. My >> > > understanding of it is as shown below. Please correct me if it’s wrong. >> > > >> > > 1. It’s not mandatory for the OS to call SetVirtualAddressMap() >> > > >> > > 2. If it’s not called, the OS can still use EFI Runtime Services in >> > > physical mode (i.e. PA == VA). >> > > >> > > >> > > >> > > For x86_64, I did a quick test with qemu and OVMF. I have commented >> > > out call to SetVirtualAddressMap() from kernel and noticed that >> > > everything works fine i.e. efi variables have the normal >> > > functionality and I ran LUV (Linux UEFI Validation). Not calling >> > > SetVirtualAddressMap() doesn’t break anything because on x86 we have >> > > 1:1 mappings. So, could someone please let me know why we might >> > > still need >> > to call SetVirtualAddressMap()? >> > > >> > >> > You are right, and the same applies to arm64. However, Apple Mac EFI >> > is known not to tolerate PA==VA at runtime, > > Some doubts again, > When SetVirtualAddressMap() isn't called, how does Apple Mac EFI realize that > the system is in runtime mode and how does it realize that it should now start > using VA instead of PA? > > Extending it further, if SetVirtualAddressMap() isn't called, any platform firmware > shouldn't be aware that the system is in runtime mode.. right? > > Or does the firmware behavior change on call to ExitBootServices()? > Yes. > AFAIK, ExitBootServices() means that boot services are no longer needed by > OS/bootloader and hence firmware can terminate them. Does it also mean that > the system is in runtime mode..? (I don't think so, as, I didn't find it in UEFI spec). > Yes > Also, could you please enlighten me on, > Why calling SetVirtualAddressMap() is optional, why not make it mandatory? > Because it depends on the OS whether it needs it or not. Also, there will always be a window after calling EBS() and before calling SVAM() where we are at runtime and no virtual mapping is installed yet, so mandating SVAM() is a bit ambiguous anyway. >> > and there may be other x86 >> > firmware implementations that don't work correctly in this case, since >> > they are only tested with MS Windows, which also calls >> SetVirtualAddressMap(). > > Regards, > Sai -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
