linux-kernel - Re: [PATCH] Allow RDTSC and RDTSCP from userspace

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Message-ID: <CALCETrWCiMkA37yf972h+fqsz1_dbfye8AbrkJxDPJzC+1PBEw@mail.gmail.com>
Date:   Sat, 25 Apr 2020 11:15:35 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Dave Hansen <dave.hansen@...el.com>,
        Tom Lendacky <thomas.lendacky@....com>,
        Mike Stunes <mstunes@...are.com>,
        Joerg Roedel <joro@...tes.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>, Juergen Gross <jgross@...e.com>,
        Jiri Slaby <jslaby@...e.cz>, Kees Cook <keescook@...omium.org>,
        kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Linux Virtualization <virtualization@...ts.linux-foundation.org>,
        X86 ML <x86@...nel.org>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [PATCH] Allow RDTSC and RDTSCP from userspace

On Sat, Apr 25, 2020 at 5:49 AM Joerg Roedel <jroedel@...e.de> wrote:
>
> Hi Dave,
>
> On Fri, Apr 24, 2020 at 03:53:09PM -0700, Dave Hansen wrote:
> > Ahh, so any instruction that can have an instruction intercept set
> > potentially needs to be able to tolerate a #VC?  Those instruction
> > intercepts are under the control of the (untrusted relative to the
> > guest) hypervisor, right?
> >
> > >From the main sev-es series:
> >
> > +#ifdef CONFIG_AMD_MEM_ENCRYPT
> > +idtentry vmm_communication     do_vmm_communication    has_error_code=1
> > +#endif
>
> The next version of the patch-set (which I will hopefully have ready
> next week) will have this changed. The #VC exception handler uses an IST
> stack and is set to paranoid=1 and shift_ist. The IST stacks for the #VC
> handler are only allocated when SEV-ES is active.

shift_ist is gross.  What's it for?  If it's not needed, I'd rather
not use it, and I eventually want to get rid of it for #DB as well.

--Andy