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[37.24.206.209]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-43970d54760sm24660629f8f.35.2026.02.23.23.28.08 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 23 Feb 2026 23:28:08 -0800 (PST) Message-ID: Date: Tue, 24 Feb 2026 08:28:07 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] xen/acpi-processor: fix _CST detection using undersized evaluation buffer To: David Thomson Cc: boris.ostrovsky@oracle.com, sstabellini@kernel.org, xen-devel@lists.xenproject.org, linux-kernel@vger.kernel.org, jgross@suse.com References: <20260223195602.17122-1-dt@linux-mail.net> 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: <20260223195602.17122-1-dt@linux-mail.net> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit On 23.02.2026 20:56, David Thomson wrote: > read_acpi_id() attempts to evaluate _CST using a stack buffer of > sizeof(union acpi_object) (48 bytes), but _CST returns a nested Package > of sub-Packages (one per C-state, each containing a register descriptor, > type, latency, and power) requiring hundreds of bytes. The evaluation > always fails with AE_BUFFER_OVERFLOW. > > On modern systems using FFH/MWAIT entry (where pblk is zero), this > causes the function to return before setting the acpi_id_cst_present > bit. In check_acpi_ids(), flags.power is then zero for all Phase 2 CPUs > (physical CPUs beyond dom0's vCPU count), so push_cxx_to_hypervisor() is > never called for them. > > On a system with dom0_max_vcpus=2 and 8 physical CPUs, only PCPUs 0-1 > receive C-state data. PCPUs 2-7 are stuck in C0/C1 idle, unable to > enter C2/C3. This costs measurable wall power (4W observed on an Intel > Core Ultra 7 265K with Xen 4.20). > > The function never uses the _CST return value -- it only needs to know > whether _CST exists. Replace the broken acpi_evaluate_object() call with > acpi_has_method(), which correctly detects _CST presence using > acpi_get_handle() without any buffer allocation. This brings C-state > detection to parity with the P-state path, which already works correctly > for Phase 2 CPUs. > > Fixes: 59a568029181 ("xen/acpi-processor: C and P-state driver that uploads said data to hypervisor.") > Signed-off-by: David Thomson > --- > drivers/xen/xen-acpi-processor.c | 7 ++----- > 1 file changed, 2 insertions(+), 5 deletions(-) > > diff --git a/drivers/xen/xen-acpi-processor.c b/drivers/xen/xen-acpi-processor.c > index 2967039..67a4afc 100644 > --- a/drivers/xen/xen-acpi-processor.c > +++ b/drivers/xen/xen-acpi-processor.c > @@ -379,11 +379,8 @@ read_acpi_id(acpi_handle handle, u32 lvl, void *context, void **rv) > acpi_psd[acpi_id].domain); > } > > - status = acpi_evaluate_object(handle, "_CST", NULL, &buffer); > - if (ACPI_FAILURE(status)) { > - if (!pblk) > - return AE_OK; > - } > + if (!acpi_has_method(handle, "_CST") && !pblk) > + return AE_OK; I understand you reflect original behavior in this regard, but why involve any ACPI function here at all when pblk is non-zero? I.e. why not swap the operands of && ? Object evaluation could have wanted side effects (in which case, however, some different change would be needed here), but checking for method presence surely hasn't. Jan