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Wed, 02 Jul 2025 05:17:09 -0700 (PDT) Message-ID: Date: Wed, 2 Jul 2025 14:17:02 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: [PATCH v7 4/7] x86: re-work memcpy() From: Jan Beulich To: "xen-devel@lists.xenproject.org" Cc: Andrew Cooper , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= References: <0b57c6f0-9fd0-4f8a-93f8-dc494a1e952c@suse.com> Content-Language: en-US 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: <0b57c6f0-9fd0-4f8a-93f8-dc494a1e952c@suse.com> Content-Transfer-Encoding: quoted-printable X-ZohoMail-DKIM: pass (identity @suse.com) X-ZM-MESSAGEID: 1751458643732116600 Content-Type: text/plain; charset="utf-8" Move the function to its own assembly file. Having it in C just for the entire body to be an asm() isn't really helpful. Then have two flavors: A "basic" version using qword steps for the bulk of the operation, and an ERMS version for modern hardware, to be substituted in via alternatives patching. Alternatives patching, however, requires an extra precaution: It uses memcpy() itself, and hence the function may patch itself. Luckily the patched-in code only replaces the prolog of the original function. Make sure this remains this way. Signed-off-by: Jan Beulich Reviewed-by: Teddy Astie Reviewed-by: Jason Andryuk --- We may want to consider branching over the REP MOVSQ as well, if the number of qwords turns out to be zero. We may also want to consider using non-REP MOVS{L,W,B} for the tail. TBD: We may further need a workaround similar to Linux'es 8ca97812c3c8 ("x86/mce: Work around an erratum on fast string copy instructions"). TBD: Some older AMD CPUs have an issue with REP MOVS when source and destination are misaligned with one another (modulo 32?), which may require a separate memcpy() flavor. --- v6: Move alternatives.c change to a separate patch. v5: Re-base. v4: Use CR2 write as serializing insn, and limit its use to boot time. v3: Re-base. --- a/xen/arch/x86/Makefile +++ b/xen/arch/x86/Makefile @@ -47,6 +47,7 @@ obj-$(CONFIG_RETURN_THUNK) +=3D indirect-t obj-$(CONFIG_PV) +=3D ioport_emulate.o obj-y +=3D irq.o obj-$(CONFIG_KEXEC) +=3D machine_kexec.o +obj-y +=3D memcpy.o obj-y +=3D memset.o obj-y +=3D mm.o x86_64/mm.o obj-$(CONFIG_VM_EVENT) +=3D monitor.o --- /dev/null +++ b/xen/arch/x86/memcpy.S @@ -0,0 +1,20 @@ +#include + +FUNC(memcpy) + mov %rdx, %rcx + mov %rdi, %rax + /* + * We need to be careful here: memcpy() is involved in alternatives + * patching, so the code doing the actual copying (i.e. past setti= ng + * up registers) may not be subject to patching (unless further + * precautions were taken). + */ + ALTERNATIVE "and $7, %edx; shr $3, %rcx", \ + STR(rep movsb; RET), X86_FEATURE_ERMS + rep movsq + or %edx, %ecx + jz 1f + rep movsb +1: + RET +END(memcpy) --- a/xen/arch/x86/string.c +++ b/xen/arch/x86/string.c @@ -7,21 +7,6 @@ =20 #include =20 -void *(memcpy)(void *dest, const void *src, size_t n) -{ - long d0, d1, d2; - - asm volatile ( - " rep ; movs"__OS" ; " - " mov %k4,%k3 ; " - " rep ; movsb " - : "=3D&c" (d0), "=3D&D" (d1), "=3D&S" (d2) - : "0" (n/BYTES_PER_LONG), "r" (n%BYTES_PER_LONG), "1" (dest), "2" = (src) - : "memory" ); - - return dest; -} - void *(memmove)(void *dest, const void *src, size_t n) { long d0, d1, d2;