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Use LEA instead of ADD when adjusting %rsp in srso_safe_ret{,_alias}()
so as to avoid clobbering flags. Drop one of the INT3 instructions to
account for the LEA consuming one more byte than the ADD.
KVM's emulator makes indirect calls into a jump table of sorts, where
the destination of each call is a small blob of code that performs fast
emulation by executing the target instruction with fixed operands.
E.g. to emulate ADC, fastop() invokes adcb_al_dl():
adcb_al_dl:
<+0>: adc %dl,%al
<+2>: jmp <__x86_return_thunk>
A major motivation for doing fast emulation is to leverage the CPU to
handle consumption and manipulation of arithmetic flags, i.e. RFLAGS is
both an input and output to the target of the call. fastop() collects
the RFLAGS result by pushing RFLAGS onto the stack and popping them back
into a variable (held in %rdi in this case):
asm("push %[flags]; popf; " CALL_NOSPEC " ; pushf; pop %[flags]\n"
<+71>: mov 0xc0(%r8),%rdx
<+78>: mov 0x100(%r8),%rcx
<+85>: push %rdi
<+86>: popf
<+87>: call *%rsi
<+89>: nop
<+90>: nop
<+91>: nop
<+92>: pushf
<+93>: pop %rdi
and then propagating the arithmetic flags into the vCPU's emulator state:
ctxt->eflags = (ctxt->eflags & ~EFLAGS_MASK) | (flags & EFLAGS_MASK);
<+64>: and $0xfffffffffffff72a,%r9
<+94>: and $0x8d5,%edi
<+109>: or %rdi,%r9
<+122>: mov %r9,0x10(%r8)
The failures can be most easily reproduced by running the "emulator"
test in KVM-Unit-Tests.
If you're feeling a bit of deja vu, see commit
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.