From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kiszka Subject: Re: [PATCH 0/7] KVM: x86: Let the guest write to multiple debug registers with one vmexit Date: Sun, 09 Mar 2014 09:15:41 +0100 Message-ID: <531C232D.40200@web.de> References: <1394192571-11056-1-git-send-email-pbonzini@redhat.com> <531C2238.1060202@web.de> Mime-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="SbL4WOktpGJsjJ3k7ohtvE6HcQnrVChD7" Cc: kvm@vger.kernel.org, alex.williamson@redhat.com, mtosatti@redhat.com, gleb@kernel.org To: Paolo Bonzini , linux-kernel@vger.kernel.org Return-path: In-Reply-To: <531C2238.1060202@web.de> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org This is an OpenPGP/MIME signed message (RFC 4880 and 3156) --SbL4WOktpGJsjJ3k7ohtvE6HcQnrVChD7 Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: quoted-printable On 2014-03-09 09:11, Jan Kiszka wrote: > On 2014-03-07 12:42, Paolo Bonzini wrote: >> Alex Williamson reported that a Windows game does something weird that= >> makes the guest save and restore debug registers on each context switc= h. >> This cause several hundred thousands vmexits per second, and basically= >> cuts performance in half when running under KVM. >> >> However, when not running in guest-debug mode, the guest controls the >> debug registers and having to take an exit for each DR access is a was= te >> of time. We just need one vmexit to load any stale values of DR0-DR6,= >> and then we can let the guest run freely. On the next vmexit (whateve= r >> the reason) we will read out whatever changes the guest made to the >> debug registers. >> >> Tested with x86/debug.flat on both Intel and AMD, both direct and >> nested virtualization. >> >> Changes from RFC: changed get_dr7 callback to sync_dirty_debug_regs, >> new patches 5-7. >=20 > This looks good now to me from KVM perspective. I was just wondering ho= w > the case is handled that the host used debug registers on the thread th= e > runs a VCPU? What if I set a hw breakpoint on its userspace path e.g.? > What if I debug the kernel side with kgdb? Ah, this part didn't change, we still switch between host and guest state on vmentries/exits. All fine! Jan --SbL4WOktpGJsjJ3k7ohtvE6HcQnrVChD7 Content-Type: application/pgp-signature; name="signature.asc" Content-Description: OpenPGP digital signature Content-Disposition: attachment; filename="signature.asc" -----BEGIN PGP SIGNATURE----- Version: GnuPG v2.0.19 (GNU/Linux) Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/ iEYEARECAAYFAlMcIy0ACgkQitSsb3rl5xQpoACaAx2oAMVCz2vgzi8BcYnXdtcW Nn0AoNKKfA4p5lRFuvFtqZTTZvf07Yk3 =G1ux -----END PGP SIGNATURE----- --SbL4WOktpGJsjJ3k7ohtvE6HcQnrVChD7--