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

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Oct 14, 2024 at 08:17:19PM +0530, Nikunj A. Dadhania wrote:
> Hi Isaku,
> 
> On 10/12/2024 1:25 PM, Isaku Yamahata wrote:
> > This patch series is for the kvm-coco-queue branch.  The change for TDX KVM is
> > included at the last.  The test is done by create TDX vCPU and run, get TSC
> > offset via vCPU device attributes and compare it with the TDX TSC OFFSET
> > metadata.  Because the test requires the TDX KVM and TDX KVM kselftests, don't
> > include it in this patch series.
> > 
> > 
> > Background
> > ----------
> > X86 confidential computing technology defines protected guest TSC so that the
> > VMM can't change the TSC offset/multiplier once vCPU is initialized and the
> > guest can trust TSC.  The SEV-SNP defines Secure TSC as optional.  TDX mandates
> > it.  The TDX module determines the TSC offset/multiplier.  The VMM has to
> > retrieve them.
> > 
> > On the other hand, the x86 KVM common logic tries to guess or adjust the TSC
> > offset/multiplier for better guest TSC and TSC interrupt latency at KVM vCPU
> > creation (kvm_arch_vcpu_postcreate()), vCPU migration over pCPU
> > (kvm_arch_vcpu_load()), vCPU TSC device attributes (kvm_arch_tsc_set_attr()) and
> > guest/host writing to TSC or TSC adjust MSR (kvm_set_msr_common()).
> > 
> > 
> > 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.

> Any links to this reported issue ?

This was not posted publically. But its not hard to reproduce.

> > Solution
> > --------
> > The solution is to keep the KVM TSC offset/multiplier the same as the value of
> > the TDX module somehow.  Possible solutions are as follows.
> > - Skip the logic
> >   Ignore (or don't call related functions) the request to change the TSC
> >   offset/multiplier.
> >   Pros
> >   - Logically clean.  This is similar to the guest_protected case.
> >   Cons
> >   - Needs to identify the call sites.
> > 
> > - Revert the change at the hooks after TSC adjustment
> >   x86 KVM defines the vendor hooks when the TSC offset/multiplier are
> >   changed.  The callback can revert the change.
> >   Pros
> >   - We don't need to care about the logic to change the TSC offset/multiplier.
> >   Cons:
> >   - Hacky to revert the KVM x86 common code logic.
> > 
> > Choose the first one.  With this patch series, SEV-SNP secure TSC can be
> > supported.
> 
> I am not sure how will this help SNP Secure TSC, as the GUEST_TSC_OFFSET and 
> GUEST_TSC_SCALE are only available to the guest.

Nikunj,

FYI:

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

At least this one:

vendor_id	: AuthenticAMD
cpu family	: 25
model		: 17
model name	: AMD EPYC 9124 16-Core Processor