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Date:   Sat, 25 Apr 2020 21:10:32 +0200
From:   Joerg Roedel <joro@8bytes.org>
To:     Andy Lutomirski <luto@kernel.org>
Cc:     Joerg Roedel <jroedel@suse.de>,
        Dave Hansen <dave.hansen@intel.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Mike Stunes <mstunes@vmware.com>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Juergen Gross <jgross@suse.com>,
        Jiri Slaby <jslaby@suse.cz>, Kees Cook <keescook@chromium.org>,
        kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        Linux Virtualization <virtualization@lists.linux-foundation.org>,
        X86 ML <x86@kernel.org>,
        Sean Christopherson <sean.j.christopherson@intel.com>
Subject: Re: [PATCH] Allow RDTSC and RDTSCP from userspace
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On Sat, Apr 25, 2020 at 11:15:35AM -0700, Andy Lutomirski wrote:
> 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.

The #VC handler needs to be able to nest, there is no way around that
for various reasons, the two most important ones are:

	1. The #VC -> NMI -> #VC case. #VCs can happen in the NMI
	   handler, for example (but not exclusivly) for RDPMC.

	2. In case of an error the #VC handler needs to print out error
	   information by calling one of the printk wrappers. Those will
	   end up doing IO to some console/serial port/whatever which
	   will also cause #VC exceptions to emulate the access to the
	   output devices.

Using shift_ist is perfect for that, the only problem is the race
condition with the NMI handler, as shift_ist does not work well with
exceptions that can also trigger within the NMI handler. But I have
taken care of that for #VC.


Regards,

	Joerg