Re: [PATCH 0/2] KVM: kvm-coco-queue: Support protected TSC

["Nikunj A. Dadhania" <nikunj@amd.com>]

Hi Marcelo,

On 10/25/2024 9:54 PM, Marcelo Tosatti wrote:
> On Mon, Oct 14, 2024 at 08:17:19PM +0530, Nikunj A. Dadhania wrote:
>> On 10/12/2024 1:25 PM, Isaku Yamahata wrote:

>>> Problem
>>> -------
>>> The current x86 KVM implementation conflicts with protected TSC because the
>>> VMM can't change the TSC offset/multiplier.  Disable or ignore the KVM
>>> logic to change/adjust the TSC offset/multiplier somehow.
>>>
>>> Because KVM emulates the TSC timer or the TSC deadline timer with the TSC
>>> offset/multiplier, the TSC timer interrupts are injected to the guest at the
>>> wrong time if the KVM TSC offset is different from what the TDX module
>>> determined.
>>>
>>> Originally the issue was found by cyclic test of rt-test [1] as the latency in
>>> TDX case is worse than VMX value + TDX SEAMCALL overhead.  It turned out that
>>> the KVM TSC offset is different from what the TDX module determines.
>>
>> Can you provide what is the exact command line to reproduce this problem ? 
> 
> Nikunj,
> 
> Run cyclictest, on an isolated CPU, in a VM. For the maximum latency
> metric, rather than 50us, one gets 500us at times.

I tried out the cyclictest after referring to the documentation[1]. Here are the
results of the run on an isolated CPU in the Secure TSC-enabled SEV-SNP VM (CPUs
16-31 are isolated in the VM):

$ sudo taskset -c 16-31 ./cyclictest --mlockall --priority=80 --interval=200 --distance=0 -q -D 5m
T: 0 ( 1226) P:80 I:200 C:1500000 Min:      6 Act:   10 Avg:   16 Max:     150
$

VM detail: 32 vCPUs VM, guest kernel v6.12-rc3 compiled with CONFIG_PREEMPT_RT=y

> 
> FYI:
> 
> SEV-SNP processors (at least the one below) do not seem affected by this problem.

Thanks for testing on SEV-SNP.

Regards,
Nikunj

1. https://wiki.linuxfoundation.org/realtime/documentation/howto/tools/cyclictest/start