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[PATCH v3] x86/entry: shrink insn size for some of our EFLAGS manipulation


  • To: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Thu, 25 Apr 2024 16:26:22 +0200
  • Autocrypt: addr=jbeulich@xxxxxxxx; 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
  • Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Thu, 25 Apr 2024 14:26:44 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

Much like was recently done for setting entry vector, and along the
lines of what we already had in handle_exception_saved, avoid 32-bit
immediates where 8-bit ones do. Reduces .text.entry size by 16 bytes in
my non-CET reference build, while in my CET reference build section size
doesn't change (there and in .text only padding space increases).

Inspired by other long->byte conversion work.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
Numbers above are biased by me also having the straight-line-speculation
change in the tree, thus every JMP is followed by an INT3. Without that,
.text.entry size would also shrink by 16 bytes in the CET build.
---
v3: Re-base.
v2: Drop switch_to_kernel change.

--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -226,7 +226,7 @@ LABEL_LOCAL(.Lrestore_rcx_iret_exit_to_g
 /* No special register assumptions. */
 iret_exit_to_guest:
         andl  $~(X86_EFLAGS_IOPL | X86_EFLAGS_VM), EFRAME_eflags(%rsp)
-        orl   $X86_EFLAGS_IF, EFRAME_eflags(%rsp)
+        orb   $X86_EFLAGS_IF >> 8, EFRAME_eflags + 1(%rsp)
 
         SPEC_CTRL_COND_VERW     /* Req: %rsp=eframe                    Clob: 
efl */
 
@@ -355,7 +355,7 @@ LABEL(sysenter_eflags_saved, 0)
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
 
         /* PUSHF above has saved EFLAGS.IF clear (the caller had it set). */
-        orl   $X86_EFLAGS_IF, UREGS_eflags(%rsp)
+        orb   $X86_EFLAGS_IF >> 8, UREGS_eflags + 1(%rsp)
         mov   STACK_CPUINFO_FIELD(xen_cr3)(%r14), %rcx
         test  %rcx, %rcx
         jz    .Lsyse_cr3_okay
@@ -370,11 +370,11 @@ LABEL(sysenter_eflags_saved, 0)
         cmpb  $0,VCPU_sysenter_disables_events(%rbx)
         movq  VCPU_sysenter_addr(%rbx),%rax
         setne %cl
-        testl $X86_EFLAGS_NT,UREGS_eflags(%rsp)
+        testb $X86_EFLAGS_NT >> 8, UREGS_eflags + 1(%rsp)
         leaq  VCPU_trap_bounce(%rbx),%rdx
 UNLIKELY_START(nz, sysenter_nt_set)
         pushfq
-        andl  $~X86_EFLAGS_NT,(%rsp)
+        andb  $~(X86_EFLAGS_NT >> 8), 1(%rsp)
         popfq
 UNLIKELY_END(sysenter_nt_set)
         testq %rax,%rax



 


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