On 6/16/22 11:44, Dionna Amalie Glaze wrote:
A question: is there some performance impact when accepting all memory under
4GB?
On SEV-SNP, we accept the HOBs 0-0xA000 and 0x10_0000-0xC00_0000,
which takes a couple seconds, which is non-negligible.
Are you possibly including the memory pinning time (that happens in the
hypervisor before the guest is started) in your measurement?
For example, I added from RDTSCP instructions in before and after the
calls to MemEncryptSevSnpPreValidateSystemRam() and got:
*** DEBUG: SEC PreValidating RAM from 820000 to 1710000
*** DEBUG: TSC1=3057966080, TSC2=3070503712, delta = 12537632
*** DEBUG: PEI PreValidating RAM from 0 to A0000
*** DEBUG: TSC1=3430088800, TSC2=3432589504, delta = 2500704
*** DEBUG: PEI PreValidating RAM from 100000 to 80000000
*** DEBUG: TSC1=3436557536, TSC2=3496324336, delta = 59766800
This is for 2GB (0x80000000) of RAM. If I've done my calculations
correctly for a 1600MHz TSC, that comes out to about 45 milliseconds.
Thanks,
Tom
For VMs that want to boot very fast and still have access to a lot of
memory in the long run (e.g., a UEFI app as an enclave or sandbox), I
admit it's not my favorite solution.
That being said, supporting unaccepted memory in the guest means that
the 4GB solution doesn't preclude an accept-on-demand solution in the
future in the case that there is demand.
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