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Wed, 12 Jun 2024 04:52:45 -0700 (PDT) Message-ID: Date: Wed, 12 Jun 2024 13:52:43 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] x86/EPT: relax iPAT for "invalid" MFNs To: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= Cc: "xen-devel@lists.xenproject.org" , Andrew Cooper References: <56063a8f-f569-4130-ac25-f0f064e288a1@suse.com> Content-Language: en-US From: Jan Beulich Autocrypt: addr=jbeulich@suse.com; keydata= xsDiBFk3nEQRBADAEaSw6zC/EJkiwGPXbWtPxl2xCdSoeepS07jW8UgcHNurfHvUzogEq5xk hu507c3BarVjyWCJOylMNR98Yd8VqD9UfmX0Hb8/BrA+Hl6/DB/eqGptrf4BSRwcZQM32aZK 7Pj2XbGWIUrZrd70x1eAP9QE3P79Y2oLrsCgbZJfEwCgvz9JjGmQqQkRiTVzlZVCJYcyGGsD /0tbFCzD2h20ahe8rC1gbb3K3qk+LpBtvjBu1RY9drYk0NymiGbJWZgab6t1jM7sk2vuf0Py O9Hf9XBmK0uE9IgMaiCpc32XV9oASz6UJebwkX+zF2jG5I1BfnO9g7KlotcA/v5ClMjgo6Gl MDY4HxoSRu3i1cqqSDtVlt+AOVBJBACrZcnHAUSuCXBPy0jOlBhxPqRWv6ND4c9PH1xjQ3NP nxJuMBS8rnNg22uyfAgmBKNLpLgAGVRMZGaGoJObGf72s6TeIqKJo/LtggAS9qAUiuKVnygo 3wjfkS9A3DRO+SpU7JqWdsveeIQyeyEJ/8PTowmSQLakF+3fote9ybzd880fSmFuIEJldWxp Y2ggPGpiZXVsaWNoQHN1c2UuY29tPsJgBBMRAgAgBQJZN5xEAhsDBgsJCAcDAgQVAggDBBYC AwECHgECF4AACgkQoDSui/t3IH4J+wCfQ5jHdEjCRHj23O/5ttg9r9OIruwAn3103WUITZee e7Sbg12UgcQ5lv7SzsFNBFk3nEQQCACCuTjCjFOUdi5Nm244F+78kLghRcin/awv+IrTcIWF hUpSs1Y91iQQ7KItirz5uwCPlwejSJDQJLIS+QtJHaXDXeV6NI0Uef1hP20+y8qydDiVkv6l IreXjTb7DvksRgJNvCkWtYnlS3mYvQ9NzS9PhyALWbXnH6sIJd2O9lKS1Mrfq+y0IXCP10eS FFGg+Av3IQeFatkJAyju0PPthyTqxSI4lZYuJVPknzgaeuJv/2NccrPvmeDg6Coe7ZIeQ8Yj t0ARxu2xytAkkLCel1Lz1WLmwLstV30g80nkgZf/wr+/BXJW/oIvRlonUkxv+IbBM3dX2OV8 AmRv1ySWPTP7AAMFB/9PQK/VtlNUJvg8GXj9ootzrteGfVZVVT4XBJkfwBcpC/XcPzldjv+3 HYudvpdNK3lLujXeA5fLOH+Z/G9WBc5pFVSMocI71I8bT8lIAzreg0WvkWg5V2WZsUMlnDL9 mpwIGFhlbM3gfDMs7MPMu8YQRFVdUvtSpaAs8OFfGQ0ia3LGZcjA6Ik2+xcqscEJzNH+qh8V m5jjp28yZgaqTaRbg3M/+MTbMpicpZuqF4rnB0AQD12/3BNWDR6bmh+EkYSMcEIpQmBM51qM EKYTQGybRCjpnKHGOxG0rfFY1085mBDZCH5Kx0cl0HVJuQKC+dV2ZY5AqjcKwAxpE75MLFkr wkkEGBECAAkFAlk3nEQCGwwACgkQoDSui/t3IH7nnwCfcJWUDUFKdCsBH/E5d+0ZnMQi+G0A nAuWpQkjM1ASeQwSHEeAWPgskBQL In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit On 11.06.2024 18:21, Roger Pau Monné wrote: > On Tue, Jun 11, 2024 at 04:53:22PM +0200, Jan Beulich wrote: >> On 11.06.2024 15:52, Roger Pau Monné wrote: >>> On Tue, Jun 11, 2024 at 01:52:58PM +0200, Jan Beulich wrote: >>>> On 11.06.2024 13:08, Roger Pau Monné wrote: >>>>> I really wonder whether Xen has enough information to figure out >>>>> whether a hole (MMIO region) is supposed to be accessed as UC or >>>>> something else. >>>> >>>> It certainly hasn't, and hence is erring on the (safe) side of forcing >>>> UC. >>> >>> Except that for the vesa framebuffer at least this is a bad choice :). >> >> Well, yes, that's where we want WC to be permitted. But for that we only >> need to avoid setting iPAT; we still can uniformly hand back UC. Except >> (as mentioned elsewhere earlier) if the guest uses MTRRs rather than PAT >> to arrange for WC. > > If we want to get this into 4.19, we likely want to go your proposed > approach then, as it's less risky. > > I think a comment would be helpful to note that the fix here to not > enforce iPAT by still return UC is mostly done to accommodate vesa > regions mapped with PAT attributes to use WC. > > I would also like to add some kind of note that special casing > !mfn_valid() might not be needed, but that removing it must be done > carefully to not cause regressions. Hmm, in the meantime I have myself sufficiently convinced that with a small (hopefully easy / uncontroversial) change to ept_set_entry() I can arrange for having the guarantees that neither INVALID_MFN nor a truncated for of it can make it into the function, allowing the check to be dropped (as you had initially asked for). >>>> One caveat here that I forgot to >>>> mention before: MFNs taken out of EPT entries will never be INVALID_MFN, for >>>> the truncation that happens when populating entries. In that case we rely on >>>> mfn_valid() to be "rejecting" them. >>> >>> The only caller where mfns from EPT entries are passed to >>> epte_get_entry_emt() is in resolve_misconfig() AFAICT, and in that >>> case the EPT entry must be present for epte_get_entry_emt() to be >>> called. So it seems to me that epte_get_entry_emt() can never be >>> called from an mfn constructed from an INVALID_MFN EPT entry (but it's >>> worth adding an assert for it). >> >> Are you sure? I agree for the first of those two calls, but the second >> isn't quite as obvious. There we'd need to first prove that we will >> never create non-present super-page entries. Yet if I'm not mistaken >> for PoD we may create such. > > I should go look then, didn't know PoD would do that. I've meanwhile checked, and indeed we do. That's what with said prereq change I hope to make no longer be the case. Jan