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Tue, 22 Sep 2026 04:54:33 -0700 (PDT) Message-ID: Subject: Re: [RFC PATCH v2 0/4] Add KVM API for confidential guest live migration From: Artem Bityutskiy To: Tony Lindgren Cc: Peter Xu , Paolo Bonzini , Sean Christopherson , Fabiano Rosas , Jon Grimm , Pankaj Gupta , Tom Lendacky , Marc Zyngier , Oliver Upton , Steven Price , Anup Patel , Samuel Ortiz , Jakub =?UTF-8?Q?R=C5=AF=C5=BEi=C4=8Dka?= , =?ISO-8859-1?Q?J=F6rg_R=F6del?= , Vishal Annapurve , Elena Reshetova , Kai Huang , Kishen Maloor , Mika Westerberg , Peter Fang , Rick Edgecombe , Xiaoyao Li , Xu Yilun , kvm@vger.kernel.org Date: Tue, 22 Sep 2026 14:54:29 +0300 In-Reply-To: References: <20260831071304.762939-1-tony.lindgren@linux.intel.com> <84bf61e0e810859ed735dc92ab94167727c2e560.camel@gmail.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.60.2 (3.60.2-2.fc44) Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2026-09-22 at 12:42 +0300, Tony Lindgren wrote: >=20 > > Anyway, if I can get the TDX module to allow dirty tracking independent= of > > migration, we could pick either mechanism, or even implement both and a= dd a > > TDX-specific ioctl to select which one to use. We could plug either met= hod > > into the KVM_DIRTY_LOG ioctl - both would work, just differently. >=20 > Note that for TDX write-blocking is an older approach. The non-blocking > SEAMCALLs were added because of the issues noticed. Both features are not > usable the same time. But let me clarify one point: write-blocking (the WP-based mechanism) isn't broken - it works fine. For fairness: - MMU-intrusiveness - valid argument. - It is "older" - not on its own a strong argument. Older does not automatically make it categorically worse. Side note: although I always try to remember that if there is a real need, we can work with the TDX module architects on the ABI to reduce that intrusiveness. I do not see the need yet, though. > If non-blocking is enabled for a TDX module write-blocking cannot be used= . > Also note that the dirty log scan features depend on non-blocking feature= s > being enabled. To make it more pronounced: Tony says the blocking vs. non-blocking TDX export mode is: - Global for all TDs, not a per-TD choice. - Decided at TDX module initialization time, and cannot be changed without re-initializing or updating the module. So in practice, it's a kernel init time decision. So my suggestion that both could be implemented and selected via a uAPI is moot in practice. Good point, thanks! Artem.