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Thu, 05 Jun 2025 03:27:45 -0700 (PDT) Message-ID: <5fd7631c-a7aa-438b-ae7e-7f35af65cef2@suse.com> Date: Thu, 5 Jun 2025 12:27:38 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: [PATCH v5 5/6] x86: introduce "hot" and "cold" page clearing functions From: Jan Beulich To: "xen-devel@lists.xenproject.org" Cc: Andrew Cooper , =?UTF-8?Q?Roger_Pau_Monn=C3=A9?= References: <73481cbf-337f-4e85-81d2-3487366cd822@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: <73481cbf-337f-4e85-81d2-3487366cd822@suse.com> Content-Transfer-Encoding: quoted-printable X-ZohoMail-DKIM: pass (identity @suse.com) X-ZM-MESSAGEID: 1749119280807116600 Content-Type: text/plain; charset="utf-8" The present clear_page_sse2() is useful in case a page isn't going to get touched again soon, or if we want to limit churn on the caches. Amend it by alternatively using CLZERO, which has been found to be quite a bit faster on Zen2 hardware at least. Note that to use CLZERO, we need to know the cache line size, and hence a feature dependency on CLFLUSH gets introduced. For cases where latency is the most important aspect, or when it is expected that sufficiently large parts of a page will get accessed again soon after the clearing, introduce a "hot" alternative. Again use alternatives patching to select between a "legacy" and an ERMS variant. Don't switch any callers just yet - this will be the subject of subsequent changes. Signed-off-by: Jan Beulich --- v5: Re-base. v3: Re-base. v2: New. --- Note: Ankur indicates that for ~L3-size or larger regions MOVNT/CLZERO is better even latency-wise. --- a/xen/arch/x86/clear_page.S +++ b/xen/arch/x86/clear_page.S @@ -5,7 +5,7 @@ #include #include =20 -FUNC(clear_page_sse2) + .macro clear_page_sse2 mov $PAGE_SIZE/32, %ecx xor %eax,%eax =20 @@ -19,4 +19,42 @@ FUNC(clear_page_sse2) =20 sfence RET -END(clear_page_sse2) + .endm + + .macro clear_page_clzero + mov %rdi, %rax + mov $PAGE_SIZE/64, %ecx + .globl clear_page_clzero_post_count +clear_page_clzero_post_count: + +0: clzero + sub $-64, %rax + .globl clear_page_clzero_post_neg_size +clear_page_clzero_post_neg_size: + sub $1, %ecx + jnz 0b + + sfence + RET + .endm + +FUNC(clear_page_cold) + ALTERNATIVE clear_page_sse2, clear_page_clzero, X86_FEATURE_CLZERO +END(clear_page_cold) + + .macro clear_page_stosb + mov $PAGE_SIZE, %ecx + xor %eax,%eax + rep stosb + .endm + + .macro clear_page_stosq + mov $PAGE_SIZE/8, %ecx + xor %eax, %eax + rep stosq + .endm + +FUNC(clear_page_hot) + ALTERNATIVE clear_page_stosq, clear_page_stosb, X86_FEATURE_ERMS + RET +END(clear_page_hot) --- a/xen/arch/x86/cpu/common.c +++ b/xen/arch/x86/cpu/common.c @@ -60,6 +60,9 @@ DEFINE_PER_CPU(bool, full_gdt_loaded); =20 DEFINE_PER_CPU(uint32_t, pkrs); =20 +extern uint32_t clear_page_clzero_post_count[]; +extern int8_t clear_page_clzero_post_neg_size[]; + void __init setup_clear_cpu_cap(unsigned int cap) { const uint32_t *dfs; @@ -357,8 +360,38 @@ void __init early_cpu_init(bool verbose) =20 edx &=3D ~cleared_caps[FEATURESET_1d]; ecx &=3D ~cleared_caps[FEATURESET_1c]; - if (edx & cpufeat_mask(X86_FEATURE_CLFLUSH)) - c->x86_cache_alignment =3D ((ebx >> 8) & 0xff) * 8; + if (edx & cpufeat_mask(X86_FEATURE_CLFLUSH)) { + unsigned int size =3D ((ebx >> 8) & 0xff) * 8; + + c->x86_cache_alignment =3D size; + + /* + * Patch in parameters of clear_page_cold()'s CLZERO + * alternative. Note that for now we cap this at 128 bytes. + * Larger cache line sizes would still be dealt with + * correctly, but would cause redundant work done. + */ + if (size > 128) + size =3D 128; + if (size && !(size & (size - 1))) { + /* + * Need to play some games to keep the compiler from + * recognizing the negative array index as being out + * of bounds. The labels in assembler code really are + * _after_ the locations to be patched, so the + * negative index is intentional. + */ + uint32_t *pcount =3D clear_page_clzero_post_count; + int8_t *neg_size =3D clear_page_clzero_post_neg_size; + + OPTIMIZER_HIDE_VAR(pcount); + OPTIMIZER_HIDE_VAR(neg_size); + pcount[-1] =3D PAGE_SIZE / size; + neg_size[-1] =3D -size; + } + else + setup_clear_cpu_cap(X86_FEATURE_CLZERO); + } /* Leaf 0x1 capabilities filled in early for Xen. */ c->x86_capability[FEATURESET_1d] =3D edx; c->x86_capability[FEATURESET_1c] =3D ecx; --- a/xen/arch/x86/include/asm/asm-defns.h +++ b/xen/arch/x86/include/asm/asm-defns.h @@ -1,5 +1,9 @@ #include =20 +.macro clzero + .byte 0x0f, 0x01, 0xfc +.endm + /* * Call a noreturn function. This could be JMP, but CALL results in a more * helpful backtrace. BUG is to catch functions which do decide to return= ... --- a/xen/arch/x86/include/asm/page.h +++ b/xen/arch/x86/include/asm/page.h @@ -219,10 +219,11 @@ typedef struct { u64 pfn; } pagetable_t; #define pagetable_from_paddr(p) pagetable_from_pfn((p)>>PAGE_SHIFT) #define pagetable_null() pagetable_from_pfn(0) =20 -void clear_page_sse2(void *pg); +void clear_page_hot(void *pg); +void clear_page_cold(void *pg); void copy_page_sse2(void *to, const void *from); =20 -#define clear_page(_p) clear_page_sse2(_p) +#define clear_page(_p) clear_page_cold(_p) #define copy_page(_t, _f) copy_page_sse2(_t, _f) =20 /* Convert between Xen-heap virtual addresses and machine addresses. */ --- a/xen/tools/gen-cpuid.py +++ b/xen/tools/gen-cpuid.py @@ -212,6 +212,10 @@ def crunch_numbers(state): # the first place. APIC: [X2APIC, TSC_DEADLINE, EXTAPIC], =20 + # The CLZERO insn requires a means to determine the cache line siz= e, + # which is tied to the CLFLUSH insn. + CLFLUSH: [CLZERO], + # AMD built MMXExtentions and 3DNow as extentions to MMX. MMX: [MMXEXT, _3DNOW],