Re: [PATCH] x86/tsx: Use MSR_TSX_CTRL to clear CPUID bits

[Pawan Gupta]

On 16.02.2022 11:46, Andrew Cooper wrote:
> On 16/02/2022 01:28, Pawan Gupta wrote:
> > On 16.02.2022 00:49, Andrew Cooper wrote:
> > > On 16/02/2022 00:39, Pawan Gupta wrote:
> > > > On 15.02.2022 20:33, Borislav Petkov wrote:
> > > > > On Tue, Feb 15, 2022 at 10:19:31AM -0800, Pawan Gupta wrote:
> > > > > > I admit it has gotten complicated with so many bits associated with
> > > > > > TSX.
> > > > >
> > > > > Yah, and looka here:
> > > > >
> > > > > https://github.com/andyhhp/xen/commit/ad9f7c3b2e0df38ad6d54f4769d4dccf765fbcee
> > > > >
> > > > >
> > > > >
> > > > > It seems it isn't complicated enough. ;-\
> > > > >
> > > > > Andy just made me aware of this thing where you guys have added a new
> > > > > MSR bit:
> > > > >
> > > > > MSR_MCU_OPT_CTRL[1] which is called something like
> > > > > MCU_OPT_CTRL_RTM_ALLOW or so.
> > > >
> > > > RTM_ALLOW bit was added to MSR_MCU_OPT_CTRL, but its not set by
> > > > default,
> > > > and it is *not* recommended to be used in production deployments [1]:
> > > >
> > > >   Although MSR 0x122 (TSX_CTRL) and MSR 0x123 (IA32_MCU_OPT_CTRL)
> > > > can be
> > > >   used to reenable Intel TSX for development, doing so is not
> > > > recommended
> > > >   for production deployments. In particular, applying MD_CLEAR flows
> > > > for
> > > >   mitigation of the Intel TSX Asynchronous Abort (TAA) transient
> > > > execution
> > > >   attack may not be effective on these processors when Intel TSX is
> > > >   enabled with updated microcode. The processors continue to be
> > > > mitigated
> > > >   against TAA when Intel TSX is disabled.
> > >
> > > The purpose of setting RTM_ALLOW isn't to enable TSX per say.
> > >
> > > The purpose is to make MSR_TSX_CTRL.RTM_DISABLE behaves consistently on
> > > all hardware, which reduces the complexity and invasiveness of dealing
> > > with this special case, because the TAA workaround will still turn TSX
> > > off by default.
> > >
> > > The configuration you don't want to be running with is RTM_ALLOW &&
> > > !RTM_DISABLE, because that is "still vulnerable to TSX Async Abort".
> >
> > I am not sure how a system can end up with RTM_ALLOW && !RTM_DISABLE? I
> > have no problem taking care of this case, I am just debating why we need
> > it.
>
> Well for one, when Linux is starting up as the kexec environment
> following Xen.
>
> You're not coding for "what logic should follow a clean microcode
> load".  You're coding for "how to take the arbitrary state that my
> preceding environment left, and turn it into what I want".

I will add the handling for this case (and I am going to follow these
words of wisdom in my future work).

Thanks,
Pawan