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Message-ID: <ZxvGPZDQmqmoT0Sj@tpad>
Date: Fri, 25 Oct 2024 13:24:29 -0300
From: Marcelo Tosatti <mtosatti@...hat.com>
To: "Nikunj A. Dadhania" <nikunj@....com>
Cc: Isaku Yamahata <isaku.yamahata@...el.com>, kvm@...r.kernel.org,
pbonzini@...hat.com, Sean Christopherson <seanjc@...gle.com>,
chao.gao@...el.com, rick.p.edgecombe@...el.com,
yan.y.zhao@...el.com, linux-kernel@...r.kernel.org,
isaku.yamahata@...il.com, Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH 0/2] KVM: kvm-coco-queue: Support protected TSC
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
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