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Mon, 11 Aug 2025 03:31:54 -0700 (PDT) Message-ID: <5af38bf6-ec55-45e1-9367-9479c2f9b298@suse.com> Date: Mon, 11 Aug 2025 12:31:53 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] RFC x86/msr: Use WRMSRNS $imm when available To: Andrew Cooper Cc: =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= , Xen-devel References: <20250808222013.1071291-1-andrew.cooper3@citrix.com> <0d78dd8e-af92-4b29-9706-e08c2c06ee89@citrix.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: 7bit On 11.08.2025 12:16, Andrew Cooper wrote: > On 11/08/2025 11:06 am, Jan Beulich wrote: >> On 11.08.2025 11:50, Andrew Cooper wrote: >>> On 11/08/2025 9:16 am, Jan Beulich wrote: >>>> On 09.08.2025 00:20, Andrew Cooper wrote: >>>>> + "mov %%rax, %%rdx\n\t" >>>>> + "shr $32, %%rdx\n\t" >>>>> + ".byte 0x0f,0x01,0xc6", X86_FEATURE_WRMSRNS, >>>>> + >>>>> + [msr] "i" (msr), "a" (val) : "rcx", "rdx"); >>>> [msr] "i" (msr), "a" (val), "c" (msr) : "rdx"); >>>> >>>> allowing the compiler to actually know what's put in %ecx? That'll make >>>> original and 2nd replacement code 10 bytes, better balancing with the 9 >>>> bytes of the 1st replacement. And I'd guess that the potentially dead >>>> MOV to %ecx would be hidden in the noise as well. >>> I considered that, but what can the compiler do as a result of knowing %ecx? >> For example ... >> >>> That said, we do need an RDMSR form (which I desperately want to make >>> foo = rdmsr(MSR_BAR) but my cleanup series from 2019 got nowhere), and >>> in a read+write case I suppose the compiler could deduplicate the setup >>> of %ecx. >> ... this. But also simply to use a good pattern (exposing as much as possible >> to the compiler), so there are more good instances of code for future cloning >> from. (In size-optimizing builds, the compiler could further favor ADD/SUB >> over MOV when the two MSRs accessed are relatively close together.) > > I have seen the compiler do this in the past, but couldn't reproduce it > for this work. > > We specifically do not want any conversion to ADD/SUB, because that > takes our "close to a nop" and makes it no so. Imo when size optimizing, size matters and "close-to-a-NOP" doesn't (as much). Jan