On 3/7/23 16:27, David Woodhouse wrote:
On Tue, 2023-03-07 at 16:22 -0600, Tom Lendacky wrote:
I did some Qemu/KVM testing. One thing I noticed is that on AMD, CPUID 0xB
EAX will be non-zero only if SMT is enabled. So just booting some guests
without CPU topology never did parallel booting ("smpboot: Disabling
parallel bringup because CPUID 0xb looks untrustworthy"). I would imagine
a bare-metal system that has diabled SMT will not do parallel booting, too
(but I haven't had time to test that).
Interesting, thanks. Should I change to checking for *both* EAX and EBX
being zero? That's what I did first, after reading only the Intel SDM.
But I changed to only EAX because the AMD doc only says that EAX will
be zero for unsupported leaves.
From a baremetal perspective, I think that works. Rome was the first
generation to support x2apic, and the PPR for Rome states that 0's are
returned in all 4 registers for undefined function numbers.
For virtualization, at least Qemu/KVM, that also looks to be a safe test.
Thanks,
Tom
I did have to change "vmgexit" to a "rep; vmmcall" based on my binutils
and the IS_ENABLED(CONFIG_AMD_MEM_ENCRYPT) change I mentioned before. But
with that, I was able to successfully boot 64 vCPU SEV-ES and SEV-SNP
guests using parallel booting.
Thanks. I'll look at retconning that rework of can_parallel_bringup()
out into a separate function, into the earlier parts of the series.
