59 lines
2.1 KiB
Plaintext
59 lines
2.1 KiB
Plaintext
DAWR issues on POWER9
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============================
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On POWER9 the DAWR can cause a checkstop if it points to cache
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inhibited (CI) memory. Currently Linux has no way to disinguish CI
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memory when configuring the DAWR, so (for now) the DAWR is disabled by
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this commit:
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commit 9654153158d3e0684a1bdb76dbababdb7111d5a0
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Author: Michael Neuling <mikey@neuling.org>
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Date: Tue Mar 27 15:37:24 2018 +1100
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powerpc: Disable DAWR in the base POWER9 CPU features
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Technical Details:
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============================
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DAWR has 6 different ways of being set.
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1) ptrace
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2) h_set_mode(DAWR)
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3) h_set_dabr()
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4) kvmppc_set_one_reg()
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5) xmon
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For ptrace, we now advertise zero breakpoints on POWER9 via the
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PPC_PTRACE_GETHWDBGINFO call. This results in GDB falling back to
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software emulation of the watchpoint (which is slow).
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h_set_mode(DAWR) and h_set_dabr() will now return an error to the
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guest on a POWER9 host. Current Linux guests ignore this error, so
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they will silently not get the DAWR.
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kvmppc_set_one_reg() will store the value in the vcpu but won't
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actually set it on POWER9 hardware. This is done so we don't break
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migration from POWER8 to POWER9, at the cost of silently losing the
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DAWR on the migration.
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For xmon, the 'bd' command will return an error on P9.
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Consequences for users
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============================
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For GDB watchpoints (ie 'watch' command) on POWER9 bare metal , GDB
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will accept the command. Unfortunately since there is no hardware
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support for the watchpoint, GDB will software emulate the watchpoint
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making it run very slowly.
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The same will also be true for any guests started on a POWER9
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host. The watchpoint will fail and GDB will fall back to software
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emulation.
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If a guest is started on a POWER8 host, GDB will accept the watchpoint
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and configure the hardware to use the DAWR. This will run at full
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speed since it can use the hardware emulation. Unfortunately if this
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guest is migrated to a POWER9 host, the watchpoint will be lost on the
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POWER9. Loads and stores to the watchpoint locations will not be
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trapped in GDB. The watchpoint is remembered, so if the guest is
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migrated back to the POWER8 host, it will start working again.
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