----- Original Message -----
> From: "Sean Christopherson" <sean.j.christopherson@xxxxxxxxx>
> To: "Paolo Bonzini" <pbonzini@xxxxxxxxxx>, "Radim Krčmář" <rkrcmar@xxxxxxxxxx>
> Cc: kvm@xxxxxxxxxxxxxxx, "Simon Becherer" <simon@xxxxxxxxxxx>, "Iakov Karpov" <srid@xxxxxxxxx>, "Gabriele Balducci"
> <balducci@xxxxxxxx>, "Antti Antinoja" <reader@xxxxxxxxxxx>, "Takashi Iwai" <tiwai@xxxxxxxx>, "Jiri Slaby"
> <jslaby@xxxxxxxx>, "Greg Kroah-Hartman" <gregkh@xxxxxxxxxxxxxxxxxxx>
> Sent: Saturday, April 27, 2019 1:38:46 AM
> Subject: [PATCH] KVM: x86: Whitelist port 0x7e for pre-incrementing %rip
>
> KVM's recent bug fix to update %rip after emulating I/O broke userspace
> that relied on the previous behavior of incrementing %rip prior to
> exiting to userspace. When running a Windows XP guest on AMD hardware,
> Qemu may patch "OUT 0x7E" instructions in reaction to the OUT itself.
> Because KVM's old behavior was to increment %rip before exiting to
> userspace to handle the I/O, Qemu manually adjusted %rip to account for
> the OUT instruction.
>
> Arguably this is a userspace bug as KVM requires userspace to re-enter
> the kernel to complete instruction emulation before taking any other
> actions. That being said, this is a bit of a grey area and breaking
> userspace that has worked for many years is bad.
>
> Pre-increment %rip on OUT to port 0x7e before exiting to userspace to
> hack around the issue.
>
> Fixes: 45def77ebf79e ("KVM: x86: update %rip after emulating IO")
The patch should probably be tweaked to use the quirks mechanism. I'll post
an adjusted version next Monday.
Paolo
