linux-kernel - Re: [RFC PATCH v3 18/59] KVM: x86: Add flag to mark TSC as immutable (for TDX)

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Message-ID: <87ilwgja6x.ffs@tglx>
Date:   Thu, 25 Nov 2021 20:40:22 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     isaku.yamahata@...el.com, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, erdemaktas@...gle.com,
        Connor Kuehl <ckuehl@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     isaku.yamahata@...el.com, isaku.yamahata@...il.com,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Kai Huang <kai.huang@...ux.intel.com>
Subject: Re: [RFC PATCH v3 18/59] KVM: x86: Add flag to mark TSC as
 immutable (for TDX)

On Wed, Nov 24 2021 at 16:20, isaku yamahata wrote:
> From: Sean Christopherson <sean.j.christopherson@...el.com>
>
> The TSC for TDX1 guests is fixed at TD creation time.  Add tsc_immutable

What's a TDX1 guest?

> to reflect that the TSC of the guest cannot be changed in any way, and
> use it to short circuit all paths that lead to one of the myriad TSC
> adjustment flows.

I can kinda see the reason for this being valuable on it's own, but in
general why does TDX need a gazillion flags to disable tons of different
things if _ALL_ these flags are going to be set by for TDX guests
anyway?

Seperate flags make only sense when they have a value on their own,
i.e. are useful for things outside of TDX. If not they are just useless
ballast.

Thanks,

        tglx