From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mail-pj1-f74.google.com (mail-pj1-f74.google.com [209.85.216.74]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 7AB1579CD for ; Tue, 14 Jul 2026 00:00:48 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.216.74 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783987249; cv=none; b=RlYOq4dn3B+z22iMiAdbm0taOiVA8SbggbeamndIRtiiW2xY8pcYDbN3hKqxmKQj1xZ+Xc/GRpIj+s0ROm5vvrFea0u30mv21kA7SfORlDLLZu5jJjVLJtGyFFBjfyZGXEm/iq3CxQxcpfp1Z50R1r2PWC/JQ8GwXj1oyZCUv3I= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783987249; c=relaxed/simple; bh=vK4nNF8Lxnnc8+sU1oNbH/TKkIaPhrbmZP8aTjmmElg=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=smAPbmtT0r0Cb2bfYyzo+2l41b6SIeYMteqIfEVOhZ3JsA5YkYqie8JZft8SR+mQmnVa56ZvfcXrIPwIv14Hj/UQkjb7IDmg76OQsoRrrYSG2gs67+1cvmd9R0r4kCTEw7YR5gE5aFEb3tvWWk0aJWmr2+9vmhHzp8MzPWU0nKg= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=q9CV34n8; arc=none smtp.client-ip=209.85.216.74 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--seanjc.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="q9CV34n8" Received: by mail-pj1-f74.google.com with SMTP id 98e67ed59e1d1-380a638fb5bso446849a91.0 for ; Mon, 13 Jul 2026 17:00:48 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20251104; t=1783987248; x=1784592048; darn=vger.kernel.org; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:from:to:cc:subject:date:message-id:reply-to :content-type; bh=60sX/LZIAI/vlaGiF0cYOJmfISm4IQZoYeFxliNrCPA=; b=q9CV34n8lnGwbU2rDX/DeAbWR/BeyG35rWlOYSfrgTBcsVbyBWlkRiXFU05w85T8ez +h+R+YUqWF2KdPBy8fXdizLWHtUVfiebhVXi5h7KjBujn2cTEjlBB7lxwix7UmDdOUGX 1AsuEITnjlYgohl1J8P2TE6ydluMz5WZgKMldVSA/D5kjTHsmBkzuzO78p/hxuCenMwg HyhzvUqhAERsjUqmNHwEtKyFw7fnqq80J6Y+YwkoszNTxIrgOC3/mEFJMC39Tht4P8Gq qlLsTizTiltgNZOIn9swp7wn9adD/7EUpOUGSn115f5qI5VnwWxiZDlC60vBrIk5fmYD TVsQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20251104; t=1783987248; x=1784592048; h=content-type:cc:to:from:subject:message-id:references:mime-version :in-reply-to:date:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to:content-type; bh=60sX/LZIAI/vlaGiF0cYOJmfISm4IQZoYeFxliNrCPA=; b=BjCx8siO9DqXoT/fnj/Kdi8GthRa8sgs7Vjosnn35oPwsi6sHc9hxJEuHIgXFi5nMv naO7+PrLvTiHQZfZgv5hJE5yE7rklMONmjMs8TyGnndR+MxEY74Irm3c+wcgm0ZSQoXN qnv8RvPwAvQLIl48v0dwz6kkRl87MmC1Bq7rGIK9qkozs9A76H7Y01/4XT9qIz44vEkD 593ROA7cJ/DoLNWXc6rt/XdUJiou4gli2VDUCP31oAAY1i/lgn8PprSRsIPYGqY2D2I7 b+4t4MZyp0sLJ+fWqQqg3wEb7T0FUjVzNFo9jE5sfBXjYRWLAhcoh8ZgBZLqlxwyxHyX EwPw== X-Forwarded-Encrypted: i=1; AHgh+RpLgASh0CW46FPwSZOUhBKn+JSYLdJJI47olPMnl59cZs+rf8/9QdS1ze0z9X4QWefttGE=@vger.kernel.org X-Gm-Message-State: AOJu0Yz0U9kraU7+I1/NgSVmRQ5HQfDZ3lB3P4qIo1jkbzq/yVR4UC0w Ec0kf8aQw+g3sLScOEEarGukTQy9trJQ/I3DD8ErZt9DTTQ7oYR5/gMepbOJL83cujUIah7Ca/1 goUYcdg== X-Received: from pjyv8.prod.google.com ([2002:a17:90a:e988:b0:37d:f943:23a]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a17:90b:4f42:b0:37f:c28a:de61 with SMTP id 98e67ed59e1d1-38dc75fa2d0mr11368740a91.17.1783987247527; Mon, 13 Jul 2026 17:00:47 -0700 (PDT) Date: Mon, 13 Jul 2026 17:00:46 -0700 In-Reply-To: Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260709204948.1988414-1-seanjc@google.com> <20260709204948.1988414-14-seanjc@google.com> Message-ID: Subject: Re: [PATCH v4 13/18] KVM: guest_memfd: Pass GPA, not GFN, to prepare() hook From: Sean Christopherson To: Ackerley Tng Cc: Paolo Bonzini , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Michael Roth , Hyunwoo Kim , Tom Lendacky , "=?utf-8?B?SsO2cmcgUsO2ZGVs?=" , Fuad Tabba Content-Type: text/plain; charset="us-ascii" On Mon, Jul 13, 2026, Ackerley Tng wrote: > Sean Christopherson writes: > >> Maybe INVALID_GPA and INVALID_GFN should be defined such that both > >> gpa_to_gfn and gfn_to_gpa are correct for both. > > > > Oh, _that_ was actually what I almost broke. I was going to track the gfn instead > > of the gpa, and then do: > > > > svm->sev_es.snp_guest_vmsa_gfn = INVALID_GFN; > > > > ... > > > > gpa = gfn_to_gpa(svm->sev_es.snp_guest_vmsa_gfn); > > > > if (gpa == INVALID_GPA) > > return; > > > > That would have failed miserably, because even if if INVALID_GFN is -1ull, the > > lower bits would be zero after the shift, i.e. wouldn't match INVALID_GPA. > > > > I _think_ make that work, by doing something like: > > > > #define INVALID_GFN ((~(gfn_t)0) >> PAGE_SHIFT) > > #define INVALID_GPA ((gpa_t)INVALID_GFN << PAGE_SHIFT) > > > > Which I actually don't mind conceptually, but I'm still not sure it's a good > > idea in practice. For whatever reason, converting between INVALID_GFN and > > INVALID_GPA just feels wrong. > > The above isn't a conversion, it's picking/defining sentinel values that > are dependent on each other :) My point is that the entire reason the sential values need to be dependent on each other is that if we add INVALID_GFN, then KVM will inevitably convert INVALID_GFN to a gpa and check the result against INVALID_GPA. > > if (to_private) { > > if (WARN_ON_ONCE(!kvm || gfn >> (kvm_host.maxphyaddr - PAGE_SIZE))) > > return -EIO; > > > > return sev_gmem_make_private(kvm, gfn, pfn, nr_pages, order); > > } > > > > Then in kvm_gmem_free_folio(), open code -1ull for the gfn: > > > > WARN_ON_ONCE(kvm_arch_gmem_convert(NULL, -1ull, > > folio_file_pfn(folio, 0), > > folio_nr_pages(folio), > > folio_order(folio), false)); > > > > Which is all a little gross? But I think it's less gross than passing a gpa, and > > we don't risk delaying this and the in-place conversion series trying to find the > > perfect answer. > > Yes thanks, we should go ahead with the GFN version, we can definitely > update this later, if at all. > > I have some more questions, though not blocking: > > + I didn't notice that NULL was passed here. Is this kind of leaking SNP > details into guest_memfd, that SNP doesn't need to know which kvm it > is to convert a page to shared? Or is it that in principle converting > to shared means "removing from ANY VM" and hence kvm = NULL? Neither? It's saying "there's no VM at this point", because there's not. And I wouldn't consider that an SNP detail, because any CoCo implementation has to work that way in practice. Requiring an active VM to reclaim memory for host usage would be unusable. > + Is the purpose of checking for the bit being set to set up a contract? No, it's to harden against against screwing things up in the futures. And the check isn't against the C-bit, it's against the max legal gfn KVM supports, which just so happens to have implications with respect to the C-bit. > It's better than passing gpa, but TDX has the opposite definition for > the bit, with 1 == shared. TDX doesn't use this callback so that's > fine, but it's hard even for SNP to check since IIUC with VTOM 1 == > shared? What if we just not check and depend only on kvm == NULL? Because the to_private path requires a valid gfn > How about this: as a lifecycle callback, guest_memfd calling this > function should be saying "guest_memfd is doing a conversion of nr_pages > beginning at this pfn to shared/private, and by the way here's the kvm > and gfn since I have the information." (all arguments presented if > available) and SNP says "I'll take action, for to_shared I don't need > kvm or gfn" (just ignore kvm and gfn for to_shared conversions) No? That's more or less what I proposed, except I guess for "by the way, here's the kvm and gfn" for to_shared. Because I do NOT think we should walk all bindings to find the kvm+gfn pairs, and because I think we should firmly establish that to_private requires a kvm+gfn pair, and to_shared does not. As above, in pracitce, that needs to hold true for any CoCo VM.